If ZipBooks only denominates in one currency, and you selected USD, then the bank account demoninated in GBP can't be in that accounting file.
It just doesn't work to have that second account display as USD when in fact it is GBP. It will make every report wrong.
This may "seem to work" with currencies with coarsely similar values (USD, CDN, GBP, Euro) but it'll fail most spectacularly if you try it with any of the penny currencies like pesos, rubles or yen. The thing is, it's failing for you equally badly, that just isn't apparent. Your books are plain wrong when they include a GBP account and call it dollars. Books that are wrong defeat the entire purpose of accounting.
Best you could do with that software is carry the account in actual USD by doing the exchange-rate conversion on all deposits and withdrawals... Then also doing occasional income/expense transactions to adjust for exchange rate changes. So for instance on that transaction, it's a dog simple $340 debit and $340 credit, and an expense for the conversion fee.