Capital Gains Tax (CGT) is normally charged at a simple flat rate of 20% when you sell shares unless they are in a CGT free wrapper such as an ISA or pension. 

If you only pay basic rate tax and make a small capital gain you may only be subject to a reduced rate of 10%. Once the total of your taxable income and gains exceeds the higher rate threshold, the excess will be subject to 20% CGT. There is also an annual CGT exemption. This means that in the current tax year you can make £12,300 of gains before paying any CGT. The allowance applies to each member of a married couple or civil partnership. 

The usual due date for paying CGT you owe to HMRC on the sale of shares is the 31 January following the end of the tax year in which a capital gain was made. This means that CGT for any gains crystalised before 6 April 2021 will be due for payment on or before 31 January 2021. However, if you waited until the start of the next tax year you would have until 31 January 2022 to pay any CGT due. For example, you could benefit from this extra year to pay CGT due by waiting to crystallise a gain from the 5 April 2021 (2020-21 tax year) until the 6 April 2022 (2021-22 tax year).  

The normal way to report a gain on the sale of shares is to complete the relevant sections of your Self-Assessment tax return. When calculating your gain, you can deduct certain costs of buying or selling shares such as stockbrokers’ fees or Stamp Duty Reserve Tax.