I contacted the DoT specifically regarding the legality of "twist and Go" on converted bikes that were made before 2016, they replied within a few days, and although they do say that only a court of law can give a definitive interpretation , i't their opinion that it's quite legal and not subject as a "one off" to type approval . they even add that it's ok, as an individual, to convert a newer used bike which would also be legal.
If it's expressed by the DoT that in their opinion something is legal to do something, I wonder who might question that ?
Anyway, here's their reply:
Dear Mr Jones
Thank you for your email of 14 November regarding your question on E Bikes manufactured before 2016 that are fitted with full speed throttles. Your correspondence has been forwarded to me to respond.
Vehicles which are powered by electric motors are considered as motor vehicles under Great Britain legislation. The Electrically Assisted Pedal Cycle Regulations 1983 (EAPC Regulations) introduced a new definition that allows certain types of electrically assisted cycles to fall outside the general legal scope of a motor vehicle and therefore not be subject to the same range of regulatory requirements. The EAPC Regulations were amended in 2015; both regulations can be found at the following links:
http://www.legislation.gov.uk/uksi/1983/1168/contents/made
http://www.legislation.gov.uk/uksi/2015/24/pdfs/uksi_20150024_en.pdf
These regulations, as amended, define an Electrically Assisted Pedal Cycle (EAPC) as a vehicle having pedals by means of which it is capable of being propelled, fitted with an electric motor that has a maximum power which does not exceed 250 watts and which cannot propel the vehicle at more than 15.5 mph. Machines that can travel under their own power (eg with a throttle) are included in the EAPC definition provided that they are capable of being propelled by the use of their pedals and conform to the other requirements listed in those regulations.
From 2016, a new EAPC, with for example a "Twist and Go" type throttle, that provides power when the rider is not pedalling, was required to be type-approved by the manufacturer before being placed on the market as an EAPC. This EU regulation had no effect on any bike that was already built and in use on the road, it only affects the manufacturing of brand new bikes.
In our understanding it is legal to take a pre-2016, "used" bike and modify it, by adding a throttle to provide power without simultaneous pedalling above 6 km/h, as long as it remains in compliance with the key requirements of an EAPC: pedals by which the cycle can be propelled, maximum power 250W, and power cuts off automatically at 25 km/h (15.5 mph). In fact, it is permissible for private individuals to modify a newer bike that has already been in use for a period as well, on condition that this is a one-off, as by definition an individual and unique vehicle
produced by a non-professional is not subject to type approval.
Please note that this email contains the views of the Department for Transport and that insofar as this letter provides an interpretation of the law, it is only a court of law that is able to give a legally definitive interpretation.
Yours sincerely