I used to have an Edge 800, after it died i tried the 820 and it was buggy as hell and the slightly smaller screen wasn’t very good
Sold it on and bought a Garmin Edge Explore for around £220, its basically a simplified version of the flagship 1030 model
It cant take a memory card but does have 16GB of internal storage and comes pre loaded with maps for the country you bought it in
DC Rainmaker is the man to listen to when it comes to Garmins, here is his review of the Edge Explore
https://www.dcrainmaker.com/2018/07/garmin-edge-explore-review.html
As for 1:50k OS mapping Ive been using Talky Toaster’s 1:50K look and feel map which costs £12
Single TT 1:50K Look n Feel Map For Self Install (GPS ONLY)
Talky Toaster’s maps have been great and Ive not noticed any real differences between OS mapping and Talky Toaster’s version