What can't I claim as a business expense?

The main things you can't claim are: personal costs such as everyday clothing, even if you wear it for work (unless it's a uniform or protective gear); entertaining clients — HMRC doesn't allow this as a deduction; fines and penalties such as parking tickets; the cost of commuting from home to your regular workplace; training that helps you move into a new field rather than improving skills for your current one; and any mixed-use costs where the personal element can't be separated. If you're unsure whether something qualifies, it's better to ask than to claim it incorrectly.