Hi.....if you want "off road" & canal paths you need (in my opinion) minimum 28/32mm tyres. Depending on how "off road" you are thinking you may want a hard-tail (front suspension) but that will add weight and is really only necessary for quite bumpy/potholey type surfaces.
I would suggest you DO NOT need 26" wheels (mountain bike) wheels as they are much harder to pedal and not required for that type of cycling. You would probably be fine with a standard 700 size wheel with 32mm tyres. If you think you are likely to go on rougher surfaces then the newer 27.5" wheels are great............a good compromise.
As the pevious poster said........
Halfords have a really good selection of bikes at that price BUT...........do you have to buy from them???? they are not renown for their service