Ignore the luddites. I’d much rather spend time looking at a map the night before and planning a route than getting a map in and out of my rucksack when I’m trying to ride. My first GPS was an Edge 305 – no maps, and a limit of 100 points on a route, and even that was a transformation when riding new routes as a quick look down confirms whether you were still on course or not, rather than having to stop to check a map.
Have a look at either the Edge 800 or 810. The only difference between the two is in the smartphone connectivity, so personally, if you can find a good price on an 800 I wouldn’t bother with the 810.
I have the OS maps, but probably wouldn’t pay for them again as the free openstreetmap ones are better, although less reliable on the legal status of trails (bw vs fp etc).