Hi,
The spot rates recorded on 31 December and 31 March, are the real-time price at which you can exchange one currency for another and are London closing rates.
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.
As long as the spot rate applies to the tax year in question, there is no preference as to which is used:
Foreign exchange: rates of exchange
Thank you.