HMRC will allow a small charge of £2 per week without query. If a larger amount is claimed then you may be asked to justify this. Detailed guidance can be found on the HMRC website at www.hmrc.gov.uk/manuals/bimmanual/bim47825.htm.
You can claim up to £104 for broadband/power and not get any queries from HMRC. If you want to add more, you'll have to do some math such as how many hours per day do you spend using those utilities for business purposes. So say if you use them 50% of the time for work and 50% for private use then you can add the yearly total and say 50% in the private use option. You may be audited though so make sure you have detailed business records at hand.
Also available: Self-Employed expenses what can I claim?
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