The reason I am not upgrading from my Edge 800 is because as far as I am concerned I've already paid for the OS maps on the SD card and there is no way I'm getting sucked into this new monetisation model until I absolutely have to. The Landranger maps are £20 for one small map on new devices!
The OS Landranger 1:25000 maps are accurate and show details down to footpaths and bridleways, As far as I’ve been able to see, the supplied map is in no way detailed enough for mountain biking in the UK and the maps that are 1:25000 are too expensive especially when you consider I have already paid for OS maps.
I'm pretty sure almost every little footpath and trail are shown, if not on the supplied maps, on free OSM maps you can install on current Edge devices. Check out OSM maps of the UK at the following site. Zoom into your riding areas to see what or what isn't depicted.