The inaugural EWR-SIN flight did use the Polar Route - maybe for a demo to the VIPs on board?
In addition I seem to recall that the first delivery of the long range A350 to SQ went from TLS and over the Pole
SQ21 now seems to frequently route over Southern England although it has also been seen going eastbound over Glasgow and Edinburgh.
When this flight operated with A345 a few years ago, it frequently oveflew the Manchester area heading eastbound.
All dependent on the upper winds as already posted
You can track the flights using Flight Aware which goes back two weeks. All these flights have used similar routes and passed over S England. Flight durations vary by up to 90 minutes from day to day
https://uk.flightaware.com/live/flight/SIA21
Tonight's is again over Southern England looking at the flight plan on the above page.