I have never been to the QF J lounge but I hear it is a bit of a zoo, but as it is the first lounge you come to (up the escalators) it could be worth looking in to see what it is like at that time?
The House - I have been in once and not for long, there was no ala carte food on offer (coming back soon, but you need to pay extra I think?). That said there was lots of space, bar service and good set of couches with an aspect over the runway - buffer food though was oddly poor - not sure just looked like it had been out all day. With hindsight I would have stayed longer, but family suggested we go to Amex lounge - based on good experiences outside of AU at Amex lounges. BA gates are normally closest to this lounge.
Amex - dungeon of a lounge, okay food, poor service, dirty plates etc and at least on the day I was there were stating clearly that it was one drink per person per hour - as the G&T was the size of a thimble this was a bit odd - and softdrink taps were off, and just had bottled water and warm fanta cans...They seem to allow almost anyone in. Not a judgmental statement, but it seemed any type of Amex got you in, and no limit on guests.

So if I was on BA in J without OWE, I would;
- check in at 90-120 mins before the flight - I am not sure what the security queues are like at the moment, and your appetite for risk.
- Assuming it is a 16.30 departure or there abouts, have a lunch in town or failing that have a lunch at the food court behind the checkin desks on the back wall before security. The options there from various Asian buffets and cook to order etc seem reasonable, better than lounge food and where the staff are often seen eating.
- Go to The House, relax on the couches, plane watch, and wait for the plane

Then enjoy my new CW 3.5 hour lunch experience on a 7 hour flight ;-)

KF