Thanks for the replies.
I should have been clearer re navigation.
I just want something for following fox for Audax. I usually follow routsheets and get lost occasionally.
The word 'just' is doing a lot of heavy lifting there.
Most cycling or outdoor GPSes will allow you to overlay a line (usually a 'track', rarely a 'route' or a 'course') on the map for you to follow, and there are plenty of smartphone apps if you want to go down that route. If you want turn-by-turn instructions to a destination (or sequence of destinations), many of the same units will do that too. But turn-by-turn instructions following a pre-set route is surprisingly difficult to achieve, mainly due to a lack of imagination on the part of Garmin's software designers back in the early 2000s.
I'm saying this as expectation management. It's possible, but it involves a learning curve and varying degrees of fucking around and pre-ride homework. Success is measured in how often it sends you off route for no obvious reason.
More generally, the units that are good at daylight visibility and battery life tend to be bad at nice big screens and scrolling/zooming the map in a responsive manner. (If you're set on using OS maps, that sort of thing becomes more important.) I'll also note that OS maps, while undeniably lovely, aren't actually that good for making sense of complicated road junctions.
Personally, I use an eTrex on the bike for telling me what to do on a pre-planned ride (eg. audax) and recording where I've been, and an OS mapping app on a smartphone/tablet for making it up as I go along when touring or mountain biking.