Savings Account Reaching $10,000

Two things:

1.  "ISA" is just a wrapper. It is what is inside that matters. If it is a cash ISA then, from the US perspective, it is a savings account where the interest is taxable and must be declared on the US tax return. If it is a stocks and shares ISA it if a whole different ball-game.

2. The exchange rate to be used is this one as at the end of the year being reported:  https://www.fiscal.treasury.gov/reports-statements/treasury-reporting-rates-exchange/

I would be concerned if, as you imply, your tax preparer has not asked questions about "foreign" assets. That is very basic stuff for a preparer working on the taxes of a US citizen abroad.

Filing the FBAR is no big deal. If you don't file them it is a big deal. If she had interest which wasn't declared, your choice is to ignore it and report it going forwards, or modify the previous returns. If it was only 12 pounds a month I know what I would do - but your mileage may vary.