This is a grey area. Assuming you are not an employee but a sole trader, things are easier. You can claim a reasonable proportion of household expenses such as light & heat, cleaning, council tax, telephone based on the space that is used for business.
The taxman will not accept a genral estimate that is not based on anything. Also make sure that you are not claiming mortgage interest, water rates, insurance as the taxman will not accept them.
I hope this helps.
Demetris Savva BA FCCA
constantinesavva@accamail.com
http://www.tax-accounting-london.info
We have an article on the subject in the FREE Special Report on How to Save Tax that we send automatically to al subscribers to our FREE weekly newsletters on tax and other business matters. You are welcome to check it out together with all the articles on our site.