I thought you had to use HMRC's exchange rates, but then got completely confuzzled because I ended up on pages talking about the exchange rate at date of remittance (?) and exchange rate at date of receipt (?) I think a lot of the information was intended for people doing business with overseas interests, but it seems the same principles would apply to basic people like us getting their pittance from US sources.
Our SS payment in ££ is the rate 'on the day' - maybe I am over simplifying it, but to me going back to average Fx rates or anything similar is WAY more than HMRC cares about for our pittance. As the SS goes in on the 3rd of the month (unless of course it's a weekend or holiday or whatever), I consider it pretty an arising Fx rate, but you could also logically say it's a remittance Fx rate. Whatever, it's not a mongo payment, and not worth worrying about. If it's being squirreled away in a US bank account, maybe different rules/thought processes will apply.
Our US income at the moment is SS and about $0.45 in interest on the a US account that we get in a year. When I start drawing RMDs from my IRA accounts in another year or so, will have to deal with that income based on my best efforts (and advice from others on how they've done it) in terms of what/how to report on the SA form. Some $$ may physically remain in the US for whatever reason, unless we need the money, but I will still consider it taxable on the arising basis, and probably use the applicable Fx rate in force on the day of 'receipt', or the HMRC rate for the month, or whatever looks right at the time. Still a few months off, and won't be a mongo amount of income anyway.