It's very quick to enter and leave Singapore to collect bags if you need to, which is likely. Security is at the gate so you enter Singapore via the e-gates, pick your bags up, check them back in at the BA desk, have a look at the Jewel, back in through the e-gates and to the lounge of your choice. Which for me would be QR - QF F is good but not a lounge to sit and relax in.