I came across much the same issue recently. A car-only search on the BA site shows the full list of Avis/Budget options. But when you search for a holiday with car hire, it only shows up a few of the categories. Couldn't work out a fix and when I called up afterwards they couldn't see the higher categories either. So I am stuck with a group C car. Oh well, let's hope I can pay for an upgrade when I pick up the vehicle.