Hi sb3502,
There are approximately 167 currencies and country zones, besides Sterling.
BIM39515 advises "Traders may use London closing rates when translating foreign currency amounts into sterling in their accounts.
Equally they may use other exchange rates, such as an exchange rate quoted by their bank, or the monthly average rates published by HMRC for VAT purposes.
You only need to query the rate of exchange used if, exceptionally, it diverges markedly from rates obtained from reputable sources of this nature".
BIM39515 - Foreign exchange: exchange rate for tax purposes
Thank you.