This is a bit complicated! The Department for Transport have produced a factsheet to help, but this is the basic situation: Road Legality: The law relating to this dates back to 1983, and covers what you can legally use on public roads in the UK.
The limits are: 200W continuous power (250W for tricycles and tandems), 15mph maximum assisted speed, maximum weight 40kgs (60kgs for tricycles and tandems).
As long as your cycle is within these limits, and you are over 14, you can ride your bike legally on UK roads with no restrictions. This law is laid down in the Road Traffic Act, and the police have the power to enforce it. Type Certification & Construction Regulations: This covers what manufacturers like us build and sell - in that respect, it’s similar to the old Kitemark or CE mark. This law has recently been revised and standardised across Europe, and is a matter for Trading Standards.
Basically, all road vehicles are supposed to be Type Certified - this is a series of standards and tests the vehicle has to pass, like an MOT, but the manufacturer can get one certificate to cover all the bikes of one model they make.
Because this process is very expensive, there is an exclusion for some electric bikes, which do not need Type Approval. To avoid it, the cycle must be under 250W continuous power, 25km/h maximum assisted speed, and must have a pedal sensor which cuts the motor when the rider stops pedalling.
In addition, the UK also has regulations about the construction of electric bikes which are used on UK roads - basically, they have to be constructed to the same standards as non-electric cycles, to comply with BS6102 part 1. So what is the situation for various bikes? Some examples: - 200W, 15mph, with pedal sensor: Completely legal to use on UK roads (and European, for that matter). Legal for the manufacturer to supply without a type approval certificate.
- 200W, 15mph, without pedal sensor: Completely legal to use on UK roads (but not European). The manufacturer should supply a type approval certificate.
- 250W, 15mph, with pedal sensor: Only legal on tricycles and tandems in the UK, NOT bicycles - a bicycle can only be used on the road with a license plate, tax, insurance, moped helmet and moped license for the rider. The manufacturer does not need to supply a type approval certificate.
- 250W, 15mph, no pedal sensor: As above, but the manufacturer should supply a type approval certificate.
- Above these limits: As above.
What about kits? The UK’s road-legality legislation still applies - so you can have 200W continuous power (250W for tricycles and tandems), 15mph maximum assisted speed, maximum weight 40kgs (60kgs for tricycles and tandems).
The whole Type Certification quagmire doesn’t apply, however - the powers that be have decided that kits are outside their remit. | 
