Hello, and welcome to FT.

Im not sure you can pre-book a seat on SQ if you have no status with them, still I would go ahead and try to call them and secure an exit row or similar type seat that you think would suit you best.

As for LHR use of lounges, your Star Gold should get you into some lounge.... I dont recall is SQ has a dedicated lounge ex-LHR, but if they do you will most probably not be given access to that. Im just transposing concepts here, since I know for sure that since you are on an economy class ticket, you will not get into the Silver Kris lounge at Changi Airport. Still, at Changi Star Golds usually get into the Premiere or SATS lounge I think its called.

Even though you seem a little sad you are in Y, Im sure you will find the SQ service in Economy to be to your liking, they are really one of the best airlines in the back of the bus - and try very hard.

Hope this helps.