If you live in a 2 bedroom house and you use 1 bedroom as an office then you can claim for 33% of you electric and gas bills, you also can claim 33% of your council tax.
This is a fact, check your local tax office, they will confirm this.
If you do not use a spare room then you can claim a weekly allowance, each tax office has a figure that they consider resonable, a lot of local tax offices will say that £3 per week is reasonable, but the £3 figure is ONLY a guildline, if you do say 20 hours a week of work in this office, would you consider this as being worth £5 a week instead of £3 that another person is only working 2-3 hours a week.
Remember that you need to justify what you claim for in your returns if they ever to a check.
And finally you cannot claim the same rate as if it was a seperate building than your home because the majority of the expensives a home incurs would happen even if you were not in business.