The following is copied straight off the HMRC website for self-employed expenses. It should give you the general principles of what you can claim and what you can't. The answer is - not much really.
"It is impossible to list all the expenses that can be deducted but, generally speaking, allowable expenditure relates to day to day running costs of your business. It includes such items as wages, rent, lighting and heating of business premises, running costs of vehicles used in the business, purchase of goods for resale and the cost of replacing tools used in the business.
Examples of non allowable expenditure are your own wages, premiums on personal insurance policies, income tax and National Insurance contributions.
Where expenditure relates to both business and private use, only the part that relates to the business will be allowed, examples are lighting, heating, and telephone expenditure. If a vehicle is used for both business and private purposes then the capital allowances and the total running expenses will be split in proportion to the business and private mileage. You will need to keep records of your total mileage and the number of miles travelled on business to calculate the correct split."