Declaring your business expenses reduces your tax bill. So it's important to capture everything that rightly should be included.
However, you can only expense costs that are needed to run your business. Personal purchases you have made with income from your business cannot be expensed for tax purposes.
The HMRC allows you to expense seven types of costs:
cost of business premises: including rent, utility bills and property insurance, as well as office expenses such as stationery, printing and computer software
travel costs: including fuel, repairs and servicing, insurance and other costs relating to the car or van you use for your business. If you are required to travel, then you can also claim any taxi, bus, train or air fares, but you cannot claim for the cost of your commute between home and work.
cost of goods that you resell: if you produce goods, or purchase them for resale, you can claim for either the cost of raw materials or the cost of purchasing the good you sold.
advertising or marketing costs: including advertisements in print or online, the cost of providing free samples of your product, and website costs
legal and financial costs: accounting, legal, insurance and other professional fees
staff costs: if you run a larger business, you can expense the salaries that you pay to your staff, including any bonuses, National Insurance and pension contributions, benefits and training courses relating to your business
clothing expenses: depending on the type of business you run, you may require uniforms or protective clothing in your business. These can be deducted for tax purposes.
For more information on allowable expenses, take a look here: What can you expense as a sole trader or side hustler?