Can I choose which coins I am selling to reduce the gain?

No. Outside the same-day and 30-day rules, every disposal draws from the pool at its blended average cost. There is no picking the expensive coin in the UK.

Does each cryptocurrency have its own pool?

Yes. HMRC's manual is explicit that each type of token needs its own pool, so your BTC, ETH and SOL each carry their own average cost.

The section 104 pool decides most of your crypto tax

Definition. The section 104 pool is HMRC’s rule that all your units of one cryptoasset form a single holding with one blended average cost. It takes its name from section 104 of the Taxation of Chargeable Gains Act 1992, and HMRC’s Cryptoassets Manual applies it to crypto at CRYPTO22200. It decides most of your crypto tax bill, and most people have never heard its name.

The idea in one paragraph. Buy 1 BTC at £20,000 and later another at £40,000, and you do not own a cheap coin and an expensive coin. You own a pool of 2 BTC that cost £60,000, an average of £30,000 each. When you sell, your gain is measured against that average, never against whichever purchase you would prefer to point at.

Worked example. Sell 0.5 BTC from that pool for £25,000.

	Amount
Disposal proceeds	£25,000.00
Pooled cost, 0.5 × £30,000	£15,000.00
Gain	£10,000.00

The pool carries forward, 1.5 BTC, £45,000 of allowable cost, and every later disposal repeats the arithmetic against the updated average.

What jumps the queue. Two matching rules apply before the pool ever gets touched. Same-day acquisitions match first under section 105, then anything bought in the 30 days after a disposal under section 106A, the bed and breakfast rule. Only what remains falls to the pool, and the order is why a quick year-end sale and rebuy usually does not do what people hope.

Also worth knowing. Each token needs its own pool, the manual says so explicitly, and NFTs sit outside pooling entirely, each separately identifiable with its own cost.

Why it is hard by hand. Every buy, sell, swap and fee changes the pool, per asset, in strict date order, across every wallet and exchange you have ever used, and one missing transaction shifts the average and changes every gain after it. gains.tax computes your pools on your own machine, shows the matching rule on each disposal, and cites CRYPTO22200 in the reports it generates, so any line can be checked against the full UK rules or HMRC’s own manual. Free below 1,000 transactions, including the reports.