It should be standard english threading. You can use any freewheel you want with that threading (the majority of them). I don't think Raleigh used the Maillard Helicomatic? Probably a 6-speed std. spaced 126mm hub?
Sunrace
IRD
Shimano
come to mind for new freewheels. The pickings in Shimano these days aren't too good for 6 speed, but the others have choices. What sort of gearing are you looking for? You'll probably want (need) to replace your chain as well. SRAM or KMC (?) 8 speed will be fine.