As Helen says. The current rate of CGT is 18% for standard tax payers, but there is an initial allowance of £10,000 and you can also take off any expenses in preparing the house for market and selling it.
ie. If you sell a house for £150,000 that cost you £100,000 and it cost you £5000.00 in expenses to sell, then you would pay 18% of £35,000 = £6,300.