The OP should prevaricate in responding to the bank. Only respond when account closure is threatened. Current accounts do not usually get closed but other financial accounts may be closed due to being a US person.
Having a Sterling mortgage as a US citizen is a big problem because of the sharp fall in the value of Sterling; but that is your issue to address rather than the bank's.
Michael Bloomberg purchased a mansion near Grenfell Towers. If he used a mortgage, he might worry about this. But for normal people with consumer loans for mortgages, cars, credit cards etc. taken out and paid in UK pounds, there is little evidence that such calculations are expected. Consumer tax preparation software used by expats does not mention it and nor do most other tax professionals.
In feudal times, the king would sell the right to tax an area to nobles for a large sum. The nobles would then often extract far more from the peasants. The practice was known as "tax farming" - it's where the word "farm" comes from. It was abandoned because it caused peasant unrest. We are now seeing the modern version of tax farming.
