UK tax question....(USD to British Pounds)

I am filling out my UK Self Assessment......(got an extension).....I'm doing the foreign income part........what's the formula for converting USD to pounds? I can't find anything on the HMRC website....and whoever sent me the form from HMRC didn't bother to send any supporting docs that give me what I need.

TIAb

You use HMRCs official exchange rate, but why are you reporting US$ on your UK return?

Are you domiciled here?  Did you remit foreign income to the UK net of US taxes?