You have to report your capital gains on the Capital gains summary form (SA100 Tax Return) when your profits are higher than the annual CGT allowance (12,000 pounds in 2019).
You will have to enter the number of disposals, profits, and losses from your crypto trades and any other capital gains you had during the year.
You can find the capital gains summary forms here: https://www.gov.uk/government/publications/self-assessment-capital-gains-summary-sa108
And with that, we conclude our crypto tax guide for the UK. We hope to have shed some light on HMRC's positions on crypto taxation for different situations.