I think the only way to achieve that, would be in conjunction with the Google Sheets data export, in Plus
Long story short, IFTTT has no way of “knowing” your balance, so you need to keep track of it somewhere.
You can set a Google Sheet up to track your balance, which updates in real time, with a cell that monitors how many £ you need to “round up” to the next whole amount.
You can then trigger IFTTT on “when google sheets changes”, and pass it that amount.
It’s a bit of faff though, but I think that’s the only viable way at the moment.