As I read it...
Utilities can be claimed against profits for tax purposes at a fraction of the number of main rooms in the house. E.g. if you have 5 main rooms (don't count bathroom/toilet/closet rooms) and you use one of them soley for business purposes then you can claim 1/5 of your council tax/gas/electricity and rent/mortgage.
When you do this you may be liable, using the same fraction, to pay a business rate of council tax. E.g. in this case 4/5 of your council tax is residential rate and 1/5 is business rate. You'd have to speak to your local council about that. They may not be bothered if it just an office.
Also note that your household insurance may not cover you if you don't declare business use.
If the room you use for business is in any way used for non-business purposes as well (e.g. if you had merely used up half an existing spare bedroom for your business and it still has the bed in it) then it was thought that you could not claim any utilities/rent/mortgage at all even if there is some business use overall. It used to be that it had to be an itentifiable part of the home used exclusively for business but it has always been possible to claim so long as the amount claimed is reasonable and propotionate to the amount used for business (you may have to agree on a weekly/monthly amount with the IR).
One way round this is to have a sofabed for overnight guests to sleep in your "office" and still look official during the day (and there are no rules as to how "plush" you can have your office)

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You can't claim the whole of your rent/utilities unless the whole property were turned over to business use.