In the UK, there are three main rules for calculating capital gains when you cash out or dispose of crypto. The rule you use depends on the timing of your transactions:

In this article, you’ll find a detailed explanation and example for each rule.

If you buy and sell the same cryptocurrency on the same calendar day, your gains or losses are calculated using the cost and sale prices from that day. In particular:

If you sell more than you bought on the same day, the excess must be calculated using the next rule.

Applying the same-day rule, the calculation will be:

Yes, your explanation is mostly correct and clear, but it can be made even more precise:

If you sell more than you buy within 30 days, the remaining amount is calculated using the next rule.

The sale on 05 July does not meet the criteria for the same-day rule, so we need to check if the 30-day rule applies. In this case, it does, because there is a purchase of ETH within the 30 days following the sale.
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The gain for the 0.5 BTC sold is based on the 10 July purchase:
(0,50 × £ 20.000,00) – (0,50 × £ 19.000,00) = £ 500,00

If neither of the above rules applies, you must use the Section 104 pool. This works like an average cost basis: you pool all your purchases together and calculate an average cost per crypto.

To calculate the gain or loss from the sale on 2 August, we need to apply the Section 104 pooling rule because:

Therefore, the calculation is as follows:

All three rules can apply to the same set of transactions, in this order: Same-Day Rule, then 30-Day Rule, then Section 104 Pool.

Let's try to calculate the capital gain/loss generated by the sale of 3,00 BTC on 30 September: