The exchange rate for the day the SS benefit is approved, and not necessarily the day of the interview, will be the exchange rate used for the WEP calculation to determine the pension. That amount becomes your official starting US SS amount in US$'s, and as the official starting amount, it is not altered. Your SS pension, with the official amount for the first year, will be increased yearly (COLA) by the inflation index, and is adjusted for inflation on (each) previous years' amount (if that makes sense), and is always in US$'s. The actual monthly payments then use that US$ amount for that year, and convert it to £'s using the prevailing exchange rate on the day of the payment (usually the 1st to 3rd of the month). Therefore, an amount you receive in one month (in £'s) will usually be different in the following months as the exchange rate varies.