This would depend on the method of accounting used. If you use the traditional accounting method, you would use the exchange rate at the time the income is earned, which is the arising basis.
Alternatively, the cash basis is a simplified accounting method, in which you would use the exchange rate at the time the money was received. You can see more information here:
Business records if you're self-employed
You also have the choice of using the spot rate or the annual average rate. Each option is acceptable. The official exchange rates can be found here:
Exchange rates from HMRC in CSV and XML format