The amounts would be a problem ...

The current method is to convert works by converting from the US value and using the exchange rate for it ...

Today, $25.00 US is £16.36 ... tomorrow it will change ...

You don't have anything that works consistently on the amounts to even guess at a method to do these conversions based on the Total based on currency rates ...

I cannot even think of a method to do this kind of conversion on the shipping modules without doing some rather intense re-writes for the calculation of the current currency ...