I can vaguely understand how they can get away with only allowing their J and F pax in to their lounge in London. But how do they get away with allowing you in by OW status when flying on another airline but not in Y?
All of QR's out-station Premium Lounges are available to both QR and Oneworld passengers, as long as they're in business class and first class. Status doesn't matter.
In fact status itself doesn't cut much ice in QR's own lounges, with the notable exception of its flagship Al Sawfa 'first class' lounge at Doha which now allows entry for Platinum members of QR's own Privilege Club frequent flyer scheme who are booked into QR business class).
For those wondering how QR can 'ban' status passengers in economy or premium economy from its lounges without falling foul of Oneworld lounge access rules: QR's 'out', in terms of Oneworld's requirement that a member airline offer lounge access to Emeralds and Sapphires regardless of class of travel, is to provide such access but to a
different lounge, instead of QR's own offering. It's still complimentary lounge access based on status, so it ticks that Oneworld box.
At LHR T4, that's the Plaza Premium lounge; at SIN T1, it's the SATS lounge; and of course, rather infamously, at DOH there are dedicated lounges labelled 'first class' (see pic!) and 'business class' which are basically for Oneworld Emeralds and Sapphires flying in economy.