If you do not have the sufficient amount of the specified currency, you can convert another currency you already hold; for example, sell EUR to buy USD.
Additionally, you can add external funds from your bank account to cover the currency conversion. Typically this will be a transfer in GBP from your GBP bank account, which you can then convert and hold as another currency in your Currency Account, i.e. USD.
You will be advised of the settlement cut-off date and time by when your external funding will need to be received, to honour the exchange rate quoted.
If funds are not available by the payment date you have set up, the payment will be held In progress until they are funded. Once funding is made, payments will be sent on the basis of the oldest payment date first. If a payment in the backlog exceeds the funds available, then this payment will be skipped and the next smaller payments will be sent.