you can only claim allowable expenses that you've actually spent, so the money has gone on those things, and isn't available to him to spend on non-job related things. You don't have profit. You have assets, but not profit
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You do know that the Inland Revenue does check books, it does ask for receipts, it does also assist you?
And lets go right back to the GOV site, so one presumes the Gov knows what its about. ... all this - and more - that you and I as PAYE don't get to claim back for - I cant claim my phone, my car, my petrol, my train fares, my work clothes, my entertainment lunch expenses, and so forth.
www.gov.uk/expenses-if-youre-self-employed
office costs, eg stationery or phone bills
travel costs, eg fuel, parking, train or bus fares
clothing expenses, eg uniforms
staff costs, eg salaries or subcontractor costs
things you buy to sell on, eg stock or raw materials
financial costs, eg insurance or bank charges
costs of your business premises, eg heating, lighting, business rates
advertising or marketing, eg website costs