Account search sums up the transaction amount regardless of currency.
British pound = 100
USD = 300
Balance on account search = 400
Use Transaction search instead of Account search.
With transaction search you can set the consolidated exchange rate to use.
1. Navigate to Lists > Search > Saved Searches > New
2. Select Transactions
3. On the Criteria> select Account is XXX
---Posting is yes
4. On the Results tab:
---Amount = Summary type = Sum
---Period = Summary type = Group
---Make sure that the Consolidated Exchange rate is set to Per-Account
---Mark the show total as Yes
5. Click Save & Run