If you have separate business premises, you can claim the full cost of utilities for that space. If you work from home, you may expense a portion of your bills.
If you're using your home as your small business premises, you can use HMRC's flat rate which varies, depending on how many hours you spend working from home:
HMRCs flat rate covers any rent and utilities. It does not cover phone or internet costs - you can expense the share of those that relates to your business.
If you have a separate business premises, you can expense all your costs for: