You really have to decide what you want the bike for. For the majority of forest and coastal cycle paths a hybrid with 32c wheels and disk brakes would be ok, just don’t expect too much if you need to hop off ledges or tackle a tree stump. If you’re talking proper off road trails then it might not be so good. The issue with a mountain bike comes if you want to go on road. Sure, it’ll be better than a road bike would be on an off road trail (as in possible to ride it), but it’ll be dog slow and it’ll handle terribly. This would be bad if you wanted to go to a rural pub following the road or pop to the shops or even commute.
If you buy a bike without figuring out where you enjoy riding, then you’ll probably see it gathering dust in the shed rather than being ridden.
I think you're right, all I do know is I have no intention of using it on a road as such although most of the path I use daily is tarmac.
I'm unlikely to use it for "proper" mountain biking any times soon ie big jumps etc. (unless I get the hang of it quickly!) but I guess a lot of it will be through woodland paths etc.
Looking at what SexyGreyFox has it looks very similar to the Trek Marlin 4 Gen 2 and that's only around £500, maybe that's worth me looking at as a starter until I know what I actually want.