The IRS says "In general, use the exchange rate prevailing (i.e., the spot rate) when you receive, pay or accrue the item." See "Translating foreign currency into US dollars".
However, if you received British pounds over the course of the year, the IRS says to use the "Yearly Average Exchange Rate". The IRS publishes these rates up to 2017 (no help for you, I guess), but I imagine that you can use a website like https://x-rates.com/ to find the exchange rate for each month and average them. The IRS will tend to accept whatever you can reasonably justify - just keep a record of how you calculated this with your tax records in case anyone ever asks.