Do you have GBP set up as an available currency on the vendor record? That could be part of the issue. (e: nvm I see the other thread now, so that's not it.)
That said, while I've never used Vendor Prepayments, I do know on Customer Payments and Vendor Payments you can only pay with a currency that matches the currency on the invoice/bill or the primary currency of the sub the doc is on. I'm guessing the same applies to VPs.
Examples:
• EUR sub, GBP Inv - can only pay the invoice using a GBP or EUR account
• USD sub, INR Inv - can only pay the invoice using INR or USD account
• CAD sub, EUR inv - can only pay the invoice using EUR or CAD account
• and so on
Unfortunately NS doesn't currently possess the "triangulation" or whatever you'd call it to deal with a third currency. Third currency being one that doesn't match the original doc or the sub default currency.