In the current tax year (6 April 2019 to 5 April 2020) you can realise capital gains of up to £12,000 without being subject to capital gains tax. If your gains are larger than that, you may have to pay capital gains tax (unless, for example, your investments are held within a ISA, or you have capital losses to offset).
The rate of tax on gains over the exempt amount is 10% or 20%, depending on your taxable income.
Last tax year (6 April 2018 to 5 April 2019) the annual exempt amount was £11,700, and the tax rates were 10% or 20% of the gain, depending on your taxable income.
At the end of the tax year we will supply you with a tax statement for any Capital Gains tax that you may be due to pay. If you don't receive a Capital Gains report this is due to the fact that you have not accrued any gains that are taxable within that tax year.