Hi,
I was blessed with one, by the wife for xmas. In answer to your questions.
1) Mine's fine for using in the Welsh mountains. I live in Aberystwyth. I can't see why the Peaks will be any different.
2) Mine's a 705 Topo so came with a maps CD. They're not OS but are fine to navigate by and aren't that different to French maps. (Well not IMO)
3) The stem mount is spot on. It clicks positively into place and kept mine in place around the Tarw at Coed y Brenin last weekend. There's not many more rocky and bumpy at a trail centre.
4) Haven't tried others. Was advised by a prominent 24hr racer that the 705 was all I needed.
5) Much less faff than paper OS maps, but you don't get HR monitor and bike computer with an OS map. On the flip side an OS map, a reasonable bike computer and an equally reasonable HR monitor could be yours for less than the cost of a 705. The difference is convenience, portability and battery life's surprisingly good.
As Dibbs says, you can't differentiate between Paths and Bridleways. Around here, it's pretty obvious the difference and a bit of common sense should be applied.
I hope I've been of some help.