In the current tax year (6 April 2017 to 5 April 2018) you can realise capital gains of up to £11,300 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 2016 to 5 April 2017) the annual exempt amount was £11,100, and the tax rates were 18% or 28% 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.