If you're self-employed, and do any work from home, you should certainly be 'renting office space from yourself' for tax purposes. Well before you need to worry about Capital GainsTax, the first thing to consider is whether you'll have to pay business rates, as well as (or partially instead of) Council Tax. The Valuation Office provides examples of when they will, and won't, regard parts of a residential address as liable for business rates, here:
http://www.voa.gov.uk...working_from_home.htm
Simply keeping a bed, wardrobe and bedside cabinet in the spare bedroom (so that its use as an office could be regarded as secondary to its prime purpose of being a guest room) might be enough to avoid having to pay business rates.
If you have to pay business rates on part of your home, you might find that this is offset by lower council tax. However your coucil tax would only fall if the valuation of the residential part of the house took the property into a lower tax band than when the whole of the house was taken into consideration. That wouldn't happen in every case.
If the Valuation Office regards part of your house as 'business premises', then it's like that HMRC would also do so when it came to considering whether CGT was also payable.
Chris