I've never heard anyone say anything bad about the Montana, but it's a bit more than I'm prepared to spend for fairly occasional use.
Quite a few of the guys in my local TRF group swear by the Road Angel 7000 Adventurer. They've been discontinued now but still crop up on eBay fairly cheap and are compatible with Memory Map.
http://www.ebay.co.uk/itm/Road-Angel...item3cccc7536d
Somewhere in the middle, I think Garmin are currently bundling full UK OS maps with the GPSMAP 62S for something like �300. You get proper buttons rather than touchscreen, but the screen is smaller.
TBH if I'm trail-riding in the UK, then I'll have paper maps of the area. You won't find trails you don't already know are there, or plan a sensible route between them without looking at a real map, or at least an electronic map on a full size PC monitor.
I do use a GPS for planned rides, particularly if I'm leading a group, because then I don't want to have to stop and check maps all the time. In that case I just create a track using my paper maps and Google Earth, and just follow the line on the screen while I'm out on the bike. I've got the paper maps with me in case it becomes necessary to detour. If I'm out on my own and just exploring, I'll work off the paper maps.
I've got a Garmin 60CSx, which is an older version of the 62S. Can't get OS maps for it even if I wanted to, only the Garmin Topo maps, which are a bit rubbish. I've just got OpenStreetMaps for the UK loaded to show me the roads and give me some sort of SatNav function on tarmac if I really needed it.