You are clearly much fitter than me
, I still manage to ride my 28kg bike over the power cut off on the flat most of the time .
Speed pedelecs are a whole new ballgame , not many suppliers in the UK .Need to be licensed, taxed , insured , require better lights , the use of motor cycle class helmets . Also , if you have more power , you need more power storage , so a mega battery or 2 to get any useful range .Out of my knowledge zone , there is a separate forum here for them , might be worth posting on that .
In your situation, as you mentioned off-road use ,think I would aim for the higher spec full suspension eMTBs , like Haibike , Cube, etc with the best Bosch/Yamaha/Shimano motors and largest battery that you are willing to shell out for . Note that they are not cheap , and will require regular maintenance for this sort of use , they put a lot of power through a bike chain and sprockets and brakes will take a hammering too .
As you sound pretty fit and are not that heavy , lower budget bikes from Whoosh, Whisper, Oxygen and Juicy might do the job, best to speak with them directly , they all have good reputations . Juicy are just up the road from me in the Peak District , which has a few bumpy bits too . They all have centre drive and hub drive options, a high torque hub might do most of what you want , but not as good off-road .