I am not sure that I follow you. If you didn’t move to the UK until April 2021 than you shouldn’t have any UK taxes to pay for the 2020-21 tax year. You will have to pay taxes for the 2021-22 tax year which just ended on April 5th.
For 2021-22, go to Settings and make sure that you have selected UK and GBP for your country and base currency. You can then select the date in April 2021 when you moved to the UK as the start of the 2021-2022 tax year. The default start of the UK tax year is April 6th so you will not be shaving many days off the tax year if you took up residency in the April 2021.
If your question concerns how to transition from a tax free jurisdiction to the UK in terms of the cost base of the crypto in your wallets/accounts when you take up UK residence, this is a UK tax issue that calls for UK tax expertise, especially when you have been trading crypto for a lengthy time prior to coming to the UK.
You may also be aware that as a non-domiciled resident of the UK, you can avoid paying UK taxes on foreign crypto or investment income by electing to file under the “remittance” approach. A UK tax adviser could help you minimize your UK taxes under this approach.
