You get 12.5k without having to pay capital gains tax. As for the other 7.5k - hypothetically, you could use a service like tornado cash to anonymously send ETH to your partner, or someone you trust, they could then cash out the 7.5 under their own CGT allowance.
You could also use p2p to sell your crypto, much harder for the banks/HMRC to sniff out but still possible. With p2p you pay a premium to sell your crypto, but it just appears as a bank transfer between two people.
Otherwise, just pay the tax on the 7.5k and be done with it.
I get what
@nikom court is saying, I prefer not to sell my crypto too but taking profits is good, if you need it for a house/car then go for it. Also good luck getting a 0% loan off the banks for 20k they just simply don't exist for 'normal' people unless you have significant assets to use as collateral