Hi,
Your client's pay you in US dollars, so you would declare the payment in US dollars in your income in the tax return, but for the purposes of the tax return, converted to GBP sterling. You should not use the sterling figure in your bank account, as the exchange to sterling is not part of your gross income. Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate. For your convenience, there are exchange rates here:
Exchange rates from HMRC in CSV and XML format
and for older rates here:
Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009
You are free to use any of the supplied rates or one of your own choosing.
Thank you.