Airport lounge is an airport lounge, and seriously there are much better places in SIN to spend time than in the lounge.

BA does not use Jewel, but still uses way way over to the left the early premium checkin area. If you are 3.5 hours beforehand then the desks will be open and the early checkin/ premium checkin will be closed, as it requires a BA member of staff.
If you arrive at 5pm, BA desks will be well open due to the flight to SYD at 7pm-ish. So just go to the BA desks will be fine, as they open just after 4pm.

Of the lounges
- QF F lounge - great for food, poor showers (pressure) and thing is about after you have 30 mins of food (they bring it out quick) what more will you do, as there no actual lounge, it is just lots and lots of tables - poor acoustics and not overly comfortable, and it gets worse as the QF flights land. It is to placate the platinum and F flyers from the poor QF food.
- BA lounge - just s standard lounge, very poor showers. Only advantage is to get the call to the flight - which can be odd given screening at the gate.
- QF business lounge - great showers and really cold beer.
- QR lounge - opens from about 6pm so depends where you are flying to and from. Excellent lounge but dark, give it 2 hours to some drinks and a nice meals and that is about it. Best showers and bathrooms though.

QR lounge is a class of travel lounge. F/J only no status pax. I do not understand the comments about being sent away, have never been sent away with a F/J BP on any OW carrier. They will scan and take a picture and huff and puff if they cannot read it with their little scanners but it is not your problem, as they just take a picture of your PNR, and work it out. It is just QR...

When I transit I give 3 hours, when I am coming form SIN I give it 1.5-2 hours.

Unless i was kicked out of my hotel, and even then I would not be that keen to get to SIN versus what I can find around Singapore.

KF