I am not sure on the exact rule, and you could always ask. Or just go to the QR lounge which as it is F/J only is a very nice lounge - and is my preferred as it is a lounge rather than a dining room.
As it is a BA F ticket you could also try the BA Concorde Bar which has been improving lately. BA typically allow access to their F lounges with an arriving flight. And email to You First would provide clarification there - they usually respond in 24 hours.