Hi,
You will need to work out what the difference is of your 12.5% disposal value, minus your 12.5% acquisition value and 12.5% acquiring and disposing costs, such as solicitors fees, estate agent fees. If the sum is greater than your annual exempt allowance, which in this tax year is £3000, then you need to report the capital gain and pay the tax within 60 days of the completion date.
You can see a calculator below, which leads on to registering for a capital gains acccount, which you can use to report and pay the Capital Gains Tax.
Capital Gains Tax
Thank you.