Ana as long as you account for it in full, it will be fine either way. But it should be declared as one lump sum (or eventually, 4 of them) so there is no need to pro-rata it across two tax years. If you were paid on May 20th 2020 then declare it in your earnings for 2020-2021.
That is presuming you are declaring your accounts as "cash basis", which you should be.
(The other option is "traditional accounting" which relies on you declaring income and outgoings on the basis of invoices that you've sent to clients, and business bills that you've received, but have not yet been paid. Which none of us will be doing for sex work!)