What's the trading allowance and should I use it instead of claiming expenses?

The trading allowance lets you deduct a flat £1,000 from your self-employed income instead of claiming your actual business expenses, which is useful if your real costs are low, as it saves you the admin of tracking receipts. If your genuine allowable expenses are higher than £1,000, though, you'd be better off claiming the actual costs instead, since you can't do both for the same income. If your total trading income is £1,000 or less before expenses, you may not even need to register for Self-Assessment for that income, though it's worth confirming your exact position with an accountant if you have other income too.