Harvey Maps are good, easier to read fine detail on the 1:25,000 than the OS equivalent due to the print quality and tones used. They're water proof, but extremely limited in coverage. Plus they're expensive, comparatively. So an OS map combined with a transparent map wallet does the job just fine, my one harvey map, Kintail/Glen Sheil, despite being water proof lives in a transparent map wallet when out and about, coz the wind isn't kind to any map, water proof or not.
OS is the way to go, unless one is after the high detail 1:12,000 detail Harvey maps for complex/dangerous mountain tops, such as the summit of the big bad ben. Even then, if your going to such places you should have all the bearings pacings from topping out of your chosen route to descent, plus alternatives, sussed out before you set out, the summit plateaux isn't the place to be figuring out your way in a whiteout when it's blowing a big hooly.