If you want to be able to follow OS Mapping, have a half decent battery life or replacable batteries and be simple to operate, the options are Garmin Oregon or Dakota (screen size is bigger on the Oregon) or Satmap, these are on offer if you follow a link provided by Midlantrailquestgraham earlier today. You will need built in OS mapping to ride off road and ideally a base map on your PC for route planning, or Where’s the path.
Only my opinion of course, I use an Oregon 450 with mapping and it’s great.