Every AI writing tool defaults to American English. That is not a minor inconvenience when you are drafting board minutes, client proposals or HMRC correspondence — it is a credibility problem. We tested Claude, ChatGPT and Google Gemini on the things UK businesses actually care about: consistent British spelling, awareness of UK-specific regulation and tax structures, and the ability to produce professional prose without sounding like a Silicon Valley press release. The results were closer than the marketing suggests, but the differences matter.
The American English Problem
AI Model Bias Toward American English
UC Berkeley research found large language models systematically favour American spelling over British equivalents
Source: BAIR UC Berkeley, Linguistic Bias in ChatGPT 2024
All three models — Claude, ChatGPT and Gemini — are trained predominantly on American English text. Research from UC Berkeley's AI Research lab found that large language models favour American spellings by as much as 43% over British equivalents. That means every time you ask an AI to write 'organisation' it has a near coin-flip chance of giving you 'organization' instead.
For a UK accountancy firm sending client letters, a consultancy drafting proposals, or a retailer writing product descriptions, this is not academic. A document peppered with 'analyze', 'center' and 'labor' signals carelessness to British readers. The question is which tool makes it easiest to stay consistently British.
How We Tested
British English Consistency Across Models
Percentage of British spelling conventions maintained across five UK business writing tasks without manual correction
Source: CTC internal testing, February 2026
We ran each tool through five practical UK business writing tasks: a client proposal for a Manchester-based consultancy, a board summary referencing Companies House filing deadlines, an employee policy document citing UK employment law, a product description for a British retailer, and a customer email referencing HMRC self-assessment deadlines. Each task was prompted with identical instructions specifying British English and UK context. We scored on three criteria: British spelling consistency, UK regulatory accuracy, and overall writing quality.
Claude Pro at £16 Per Month
Claude Pro costs $20 per month — roughly £16 at current exchange rates. An annual plan brings that to about £14 per month (£168 upfront). Anthropic also offers a Team plan at $25 per user per month billed annually.
On British English, Claude was the steadiest of the three. Once instructed to use British spelling, it held that standard across long documents with fewer lapses than ChatGPT or Gemini. Where it stood out was in long-form business writing — revision cycles, restructuring paragraphs, and maintaining a consistent tone across multi-page documents. Claude also handled UK regulatory references well, correctly naming Companies House filing windows and HMRC self-assessment deadlines without hallucinating dates.
The weakness: Claude's knowledge cutoff means it can miss very recent regulatory changes unless you paste in the relevant text. It also lacks native web search in the free tier, which limits fact-checking during drafting.
ChatGPT Plus at £20 Per Month
ChatGPT Plus costs $24 at checkout in the UK — roughly £20 including VAT. OpenAI also offers a Business plan from two users upward and a Pro tier at £200 per month for heavy users.
ChatGPT is the dominant AI writing tool in the UK and globally. Its strengths are breadth and speed — it handles creative writing, brainstorming, and conversational copy better than either competitor. The built-in web browsing means it can check current information during a drafting session.
The British English problem is real, though. OpenAI's own community forums are full of requests for a default British English setting. ChatGPT requires users to specify British English with each new session or prompt, and even then it slips back to American conventions — 'specialize' instead of 'specialise', 'analyze' instead of 'analyse' — particularly in longer documents. The custom instructions feature helps but does not eliminate the problem entirely.
On UK regulatory context, ChatGPT performed adequately but occasionally confused UK and US tax terminology, referencing 'fiscal year' where 'tax year' would be correct for HMRC purposes, or using 'LLC' in contexts where 'Ltd' was appropriate.
Google Gemini from £5.90 Per Month
Google Gemini is included in all Google Workspace plans — Business Starter at £5.90 per user per month, Standard at £11.80, and Plus at £18.40, all billed annually excluding VAT. For businesses already on Google Workspace, this makes Gemini the cheapest option because there is no additional charge for AI access.
Gemini's strength is its deep connection to the Google Workspace tools UK businesses already use. It works inside Docs, Sheets, Slides and Gmail without switching applications. For businesses that live in Google Workspace, this matters more than any benchmark score.
On British English, Gemini fell between the other two. It was better than ChatGPT at maintaining British conventions when prompted, but not as reliable as Claude across longer documents. The Gemini side panel in Google Docs can summarise, restructure and refine text, and its grammar suggestions are competent. A June 2025 test confirmed Gemini 2.5 Pro could convert informal writing to formal British English without altering meaning.
On UK regulatory awareness, Gemini benefited from Google's search grounding — it could pull current information from the web mid-conversation. But its base knowledge of UK-specific business context was the weakest of the three, occasionally producing generic advice that could apply to any English-speaking country.
The Pricing in Context
Annual Cost Comparison for a Solo UK Professional
What each AI writing tool costs per year for a single UK user, converting USD pricing to GBP at current rates
Source: Anthropic, OpenAI, Google pricing pages February 2026
For a five-person UK business, the annual cost difference matters. Claude Pro for one user costs roughly £192 per year. ChatGPT Plus costs about £240. Gemini via Google Workspace Starter for five users costs £354 per year — but that includes email, storage, Docs, Sheets and video conferencing, not just AI. Comparing like for like on AI writing alone, Claude is the cheapest standalone option. But if your business already pays for Google Workspace, Gemini costs you nothing extra.
Where Each Tool Wins
Claude is the strongest choice for long-form UK business writing — contracts, proposals, board papers, policy documents. It holds British English with the fewest lapses and handles revision cycles without drifting. Pick Claude if writing quality and British consistency are your top priorities.
ChatGPT is the best generalist. It handles the widest range of tasks — creative copy, brainstorming, customer emails, social media — and its web browsing gives it an edge for research-heavy writing. Pick ChatGPT if you need one tool for everything and can tolerate occasional American spellings.
Gemini is the smart choice for Google Workspace businesses. The workflow connection alone — writing inside Docs, drafting in Gmail, analysing data in Sheets — saves time that standalone tools cannot match. Pick Gemini if your team already lives in Google Workspace and values workflow speed over writing perfection.
What None of Them Do Well
All three struggle with highly specialised UK terminology — Construction Industry Scheme references, NHS-specific procurement language, or Agricultural Holdings Act clauses. None of them reliably format documents to UK business standards (A4, UK date format, £ currency) without explicit instruction. And all of them occasionally hallucinate UK-specific details — a Companies House filing deadline that is off by a week, an HMRC threshold from two years ago.
The honest conclusion: if you need error-free UK regulatory content, no AI tool replaces a final human review. These tools accelerate drafting, not replace checking.
A Practical Decision Framework
If you are a solo professional or micro-business, start with Claude Pro at £16 per month for anything client-facing, and use the free tiers of ChatGPT and Gemini for quick tasks. If you run a five-to-twenty person team already on Google Workspace, use Gemini for day-to-day work and add one Claude Pro licence for whoever handles formal documents. If you need creative output and research capability above all, ChatGPT Plus gives you the broadest toolkit.
The strongest approach is not picking one tool — it is knowing which tool fits which task. British businesses using multiple AI tools strategically will outperform those locked into a single platform.
