Since you've never lived in it, it has never been your principal private residence, so any capital gain has a CGT liability, less capital improvements you have made. Most of the £20k can probably be counted as capital improvements but it boils down to specifics. Any net gain will have your annual CGT allowance deductible, just over £10k, then you pay CGT on the residue.
Presumably you've been declaring any net income from rent as income for income tax, though perhaps there isn't any.