Originally Posted by
KARFA
The easiest way to do this is book the WTP fare online using the voucher, then call to do the UuA. Use the online tool to UuA in MMB with caution, it can have a tendency to overcharge.
Cheers for the info, so to clarify I book the underlying fare and then call the avios line to UuA, They did answer the phone and didn't cut me off but couldn't do a purchase and UuA only an Avios purchase.