Many thanks for your replies.

The voucher for 2 flights to Miami (economy as that's all the voucher will let me select due to the changes in the companion vouchers) is £200 each in taxes.

If I thought there would be a POUG offer in MMB to WTP I could then upgrade to busineess at the desk on the day. Or, with airmiles if it would let me over the phone as suggested.

It's the not knowing whether we would be denied an upgrade to business from economy at the desk on the day if nothing comes up in MMB before hand!