Currency conversion for reporting Capital Gains for US stock

I bought and sold US stock using my US dollar saving. When reporting Capital Gain, please advise: 1. I can use the HMRC's monthly currency conversion rate at the selling date to calculate the purchasing price / disposal proceed / gain in Sterling? or 2. I need to use the HMRC's monthly currency conversion rate at the purchasing date to calculate the purchasing price, and use the monthly currency conversion rate at the selling date to calculate the disposal proceed? Please advise whether both are acceptable? or if not, which one should be used?

Hi
Number 2 would apply but you can use any rate that is beneficial to you within the particalar tax year of purchase and sale.