The car hire is the reason no upgrade is being offered. Well it's not so much they are not upgradeable but they may well be using the G fare bucket which doesn't work like other revenue bookings. If it is a BA Holidays booking then you can call up BA Holidays for a quote, alternatively if it was a car hire added to an existing booking (in which case you won't be in G) then you can call BA on the usual Contact Centre number. In either case it's too complex for the self serve aspect of MMB to calculate the upgrade price.

In these cases it will almost certainly cost you more than if you had booked the higher cabin at the original purchase. Pro-active upgrade offers (POUGs) - which are also not necessarily cheaper if you are spending the weekend away either - will only be offered on fairly simple single or return trips.

However you can still do an airport upgrade (AUP). Prices for AUP and POUG are in the Forum Dashboard.