I use an Oregon 600 with both OSM and OS maps (selectable).
It will actually support both HRM and Cadence devices if you want that too.
Great for multi-day stuff as it uses AA batteries.
There’s an option to buy it with a Garmin rechargeable battery pack. This is just two (low capacity) NIMHs in a wee case that also depresses an internal button so it can be charge in situ. Use your own higher-capacity NIMHs and a wee bit of card over the button to achieve the same thing.
The screen can sometimes react to water (or sweat) falling on it. I always run in power save mode and just remember to wipe it with a glove (or something) before using it.
The bar mount is OK but I’d advise using a lanyard too, just in case you haven’t pushed it home properly and it falls off when riding.