The rule behind every claim
An expense is allowable if it is incurred wholly and exclusively for the purposes of your trade. That does not mean it must be solely for business, but it must be genuinely for the business. Where an expense serves both, only the business portion can be claimed, such as the working-use share of home broadband or a mobile bill.
Capital purchases work differently: rather than deducting the full cost, you normally claim capital allowances, for example the main-rate writing-down allowance of 14%. Vehicles, tools and equipment generally fall here rather than in expenses.
Common claims, and common traps
Typical allowable expenses include stock and materials, business insurance, advertising, accountancy fees, business travel and subsistence, workspace costs, and business-use portions of household bills. Uniforms and protective clothing count; everyday clothing that you would wear anyway does not.
The traps are usually meals (only claimable in specific circumstances, such as overnight business travel), commuting (not claimable), and entertaining clients (never allowable). Clothes for a freelancer's on-camera work, haircuts, and gym memberships are regular refusal cases.
Keep the evidence as you go
Every claim should be backed by a receipt, invoice or bank record. HMRC can look at your records well after you have filed, and the burden of proof sits with you. Capturing receipts at the point of purchase, rather than reconstructing them from a bank statement in January, is the difference between a defensible return and a nervous one.