I wrote the tool to handle year-end accounts, but you can run it monthly if you like.
All the tool does is apply a currency adjustment journal, as described in the “Journal out a currency loss/gain in a foreign bank account” section of Invoicing in foreign currencies by looking up the exchange rate on the relevant dates.
It ensures that the GBP figures shown in the Chart of Accounts and other reports matches the current GBP value of foreign currency accounts on the specified date.