South Downs way, end-to-end, TBC

What gear ratio though?

As much as I'm just trying to wind people up, I honestly want to have a go at some point

Beauty of the SDW is that you can split it up into chunks which reference the nearest train station, and each logical chunk contains a really nice mix of terrain and scenery.

This is a nice route to do (it's the first bit) if you wanted to come back to Winchester to for e.g. pick up your car: https://www.strava.com/activities/2343776855

That does contain the clay section however, which I'd swerve.

Any terrain is possible on singlespeed, so it's worth attempting. The challenge is that you'll either be walking for large chunks of it due to being over-geared, or make very slow progress due to being under-geared.