It’s very important to understand that your household costs are not part of your home business. Unfortunately you will not get any existing household costs past the HMRC. This means that any costs you would have had without your office being on your premises will just not fly. Think of your business as being stand-alone from your property regardless of where it is so phone line and internet rental, mortgage, council tax etc. are all attached to your property regardless of where you work.
If you have a dedicated phone line and internet connection this is tax deductible. This can all be quite confusing to us but your accountant can make clear exactly where you stand and where the line is between your home and your office.
If you work from home but for an employer (rather than running your own business) there are claims you can make depending on the reasons and the time you spend there. The HMRC website can tell you more.