One of the best for derestricting is the Oxygen MTB. It can do about 24 mph and has lots of power for hills. It's easy to derestrict using a secret button sequence. If you get one, I'll tell you how. It has good tyres and hydraulic brakes, which are necessary if you want to go that fast. They were at the Cycle Show last week offering a very good discount (£200 off), which you could probably still get if you order soon. The discount was offered by Ebikes direct, but you should also be able to get it from South Yorkshire Electric bike Centre, who are the distributors, if that's nearer to you.
http://www.e-bikesdirect.co.uk/Item/Brands/Oxygen_Electric_Bikes/Oxygen_Emate_MTB_13AH.aspx
The Kudos Tornado is another good one. It's a bit cheaper, but with slightly less range. The Cyclotricity one is also fast; however, the big motor is heavy and not so efficient as the lighter geared motors. Expect the battery to go down very quickly if you use the available power.
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