Sale of crypto for fiat currency (e.g. BTC --> GBP) is a taxable and reportable event. The capital gains/losses can be calculated by subtracting the cost basis and the associated fees from the proceeds. You may refer to the examples below illustrating the share pooling method.
Example 1:
You made below transactions on the respective dates:
Buy 10 ETH for GBP 10,000 on 1 Feb 2020
Buy 5 ETH for GBP 8,000 on 5 Feb 2020
Buy 4 ETH for GBP 8,000 on 10 Aug 2020
Sell 5 ETH for GBP 10,000 on 10 Aug 2020
Results:
Since there is a purchase on the same day of disposal (i.e. 10 Aug 2020), we should deduct the cost basis of the same day first. If there are no purchases within 30 days after the disposal, we can take the weighted average from the section 104 pool to calculate the cost basis.