Twist and go ebikes were legal up to 2016 to sell and still legal to use if you have one. If you have a bicycle made before that cut off date and fit a twist and go ebike kit of 250W and 15.5mph maximum assistance I still wonder if that is legal as ebike kits were never integrated into law but have been accepted as legal it seems. There is still a procedure for having a twist and go ebike without insurance and road tax etc but they have to be individually tested and of course aren't legal for use outside the UK. Twist and go ebikes allow for the elderly and disabled to cycle it was a retro step when we implemented the European laws here that restricted ebikes to pedal assist only. Many of the mid-drive motors have peak output well above 250W some close to 800W like some Bosch models. I think the big issue is very powerful motors that assist beyond 15.5mph. While assisting I can't see the need to pedal. If you are not pedalling it helps you concentrate on what is happening around you which is important in busy urban traffic.
Even the government site has a confusing information.
https://www.gov.uk/electric-bike-rules
They state maximum power of 250W but of course almost all ebikes exceed 250W. 250W is normally nominal or rated power. I think the correct definition is the maximum power the motor can hold without over-heating and can therefore provide that power continuously so at 251W it would start to over-heat and could only do that for shorter periods. However often its the same motor sold at 250W and 350W just electronically restricted at 250W and perhaps peak power twice that amount. Most e-mountain bikes with mid-drive motors are technically illegal as often exceed well over 500W power but no one cares it seems. If you removed all ebikes from the road with questionable legality probably most would disappear. Mid-drive motors have the advantage that the controller circuit and motor are contained within the same unit so can have a very high current channel to provide very high currents hence the 800W power. People think because its a £8k ebike sold in a shop somehow its more legal but they are also pushing beyond the boundaries of ebike law.
Must be a absolute nightmare for the police to understand and lets face it all the evidence I've seen is they don't bother. Yes they will get the hugely powerful 2000W ebikes and crush them on occasion but most of the time nothing happens. I mean really the only thing they can check for is the 15.5mph/25kph assistance speed easily which actually is a 17mph maximum assistance speed allowing for the legal tolerance.
On a hub motor ebike if the chain breaks the motor still operates when you turn the cranks despite the person not assisting the bike at all. You are just spinning the cranks with zero resistance. There is no law to stop you removing the chain and operating your ebike this way. You are still technically pedalling to allow the motor to function.
The way ebike law is written is a complete mess.
Something simple like maximum ebike weight before additional accessories fitted 30kg and maximum assistance speed 15.5mph would be so much more sensible. Maybe a maximum acceleration limit would be useful too to improve safety especially on shared paths. Maybe a braking maximum distance would help too. We want to get as many people cycling as possible so I feel wattage should not be restricted for heavier riders especially elderly or disabled who need maximum assistance for hills.