The Information Commissioner's Office (ICO) has guidance on…
When running a business from home you can claim part of your household expenses as a business expense.
You are allowed to deduct the part of your expenses that wholly, exclusively and necessarily relates to costs incurred to run your business from home. If you have converted a room in your house into an office, then you can claim certain expenses based on either the percentage floorspace used for or the amount of time it is used for business purposes.
You can then, for example, use the percentage of floor space used for business purposes to calculate the portion of your rent, council tax, mortgage interest, insurance and water rates that is business related. The HMRC provides detailed information on their website about which expenses you can deduct if you are running your business from home.