@davidr I've had such posters for the past 10 years and they helped me get inspiration.

Quite easy to make with #MapOSMatic https://print.get-map.org/

If you want to jump down the rabbit hole, make your own style with #TileMill https://tilemill-project.github.io/tilemill/

The only difficult part is installation. Then creating the style is fun, but still lots of work.