Welcome to Flyertalk and welcome to the BA Forum.

As things stand BA84 is indeed the A380, the double decker aircraft, and if you book World Traveller Plus (premium economy) you can only be sat on the upper deck - that's where all that cabin's seats are to be found. It's also arguably one of the best premium economy products out there, with the window seats having access to a very useful side storage unit along the fuselage. Two things to be aware of, firstly that equipment can change and if that happens there isn't a lot you can do about it. You didn't mention the date of travel so I can't check that for you, but in any event there could be a last minute swap. Secondly A380s have a bit of a reliability problem. It seems to be getting better but the pademic meant the aircraft didn't fly, and that doesnt appear to have done them much good.