Quote from SSi on 2026-05-28, 9:30 am

Your transaction download .csv files will usually have the credit/debit amount in £, but some investors can choose to work in $USD.
The .csv import function for ii files, no accepts both £ and $-based reports.

The $ is assumed to be USD and each buy/sell price will be converted to GBP using the historical Fx rate on the transaction date.