rear wheel 126mm spacing

When rear dropout spacing was 120mm OLD which used to be pretty much 3 speed hub gears and 5 speed freewheels they decided to create a new rear dropout spacing of 126mm to accommodate 6 speed freewheels with the same cog spacing as 5 speed just with an extra gear so no reduction in gear changing reliability but when they created 7 speed freewheels they reduced the cog spacing so pretty much the same dropout spacing could be used hence why 7 speed freewheels are often not quite as good at shifting as 6 speed. I'm guessing the 7 speed freehub is also on the same dropout spacing as this. So what you are basically requesting is a rear wheel for either a 6/7 speed freewheel or a freehub limited to a maximum of 7 gears. I think if you are hoping to use a 8 speed or more cassette you will need to increase the dropout spacing.

Alternatively you can get more closely spaced freewheels with 8, 9 or 10 gears but that is a poor quality option with lower shifting reliability and weaker parts and they may still require larger dropout spacing anyway I'm unsure about that.

http://www.sunrace.com/en/products/freewheels

Looks like Malcolm has them:
https://thecycleclinic.co.uk/products/z ... s-son-rims