If you are basing your purchasing decision based on LHR lounge quality, go for QR for F&B and imho a superior lounge - food cooked to order etc., or EY, which doesn't have quite as good F&B (although decent service with wandering wait staff) as it's mainly a buffet supplemented by some order options as opposed to a la carte restaurant style of QR, but has a decent little spa if that's your thing.
EK's offering before and after the renovations will be the same as their global outstation offering, so more of a very nice J class facility rather than quasi-F that EY and QR have at LHR (quasi-F being the dine-on-demand options and complimentary spa treatments).
The BA J offering at LHR T3 is, compared to EY, QR and EK, poor. Although I think in the grand scheme of lounges, perfectly adequate, but you did give only 4 options. Otherwise we could go on about the VS Clubhouse or the new LHR T2 facilities
You can in fact find pictures and reviews of all of these lounges in the Trip Reports forum, or failing that, Google.