Tax on staking, mining and DeFi income
Not all crypto tax is capital gains. Rewards you earn — staking, mining, some DeFi yield, certain airdrops — are usually taxed as income first, and then capital gains applies a second time when you eventually dispose of those coins.
Income at the point of receipt
Staking rewards and most DeFi returns are generally treated as miscellaneous income, valued in GBP at the market price on the day you received them. Mining is miscellaneous income if it is a hobby, or trading income if it amounts to a business.
That GBP value at receipt is taxed as income — and it also becomes the acquisition cost of those new coins for capital gains purposes.
Capital gains on the second leg
When you later sell or swap reward coins, you have a normal CGT disposal. Your cost basis is the value you were already taxed on as income, so you are not taxed twice on the same amount — only on any further movement in price between receipt and disposal.
Airdrops are a special case
An airdrop received in exchange for doing something (a service, a promotion) is income. An airdrop received with no strings attached may not be income at receipt — but it is still subject to CGT when you dispose of it, with a cost basis that may be nil. Keep the date and GBP value of everything you receive.