You've been given good advice here about the law and electric bikes in the UK. If you want to buy an offroad kit and use it with your town bike do so by all means, but expect it to be impounded if the police catch you using it. They will be cracking down on this issue, I suspect - it's in the public eye now.
Police stop every bike converted to electric?
Also, for town riding, I can (from what I read) reprogram the controller to 250W and the speed on the regular display.
Anyway, I have no intention of going over 20mph on my bike! Especially in the town where pothole upon pothole.
This is supposed to be for off-road.
It's also meant to be for later, to change bikes, maybe to fat tyres.
I don't want to buy a set again for another £500 or £800.
I want to buy once to last 'forever'. Especially as it's a very simple fitting, not interfering with the structure (no drilling holes).