Another option I used for a few trips was a used Nuvi from 50 quid.
I doubt a Zumo worth using ever gets that cheap. Took Olaf with no bother and maybe OSM too.
Only thing is they are fragile and not weatherproof, but you can buy waterproof cases/mounts on ebay for a tenner and the screen is good and big.

Since I started using GPS in North Africa the advent of GPS mapping (as opposed to waypoints on a blank screen) has changed the game.

Prices being equal, I'd sooner get a used Montana over a Zumo - a true do-it-all satnav/GPS.