In reply to Mark Edwards:
> One advantage to a conversion is that button only operation is an option as the regulations only apply to manufactured ebikes.
Not strictly correct. The "Kit Motors" section here: https://www.pedelecs.co.uk/forum/threads/pedelec-law-the-details.37594/ states:
...both individuals and suppliers operate in a legal vacuum in which all* try to supply and/or use as closely conforming to the complete e-bike law as they can, trusting that is acceptable. That has always worked throughout all of Europe and the UK without any mention of a possible prosecution, providing the three main points of the law are adhered to, i.e. 250 watts maximum assist, 15.5 mph maximum assist speed and power only when pedalling.
It then goes on to explain how one might go about ensuring that one's home-brewed e-bike definitely is a road legal EAPC, by following the Single Vehicle Approval (SVA) process. What's particularly relevant to the question of having a throttle is that it goes on to state:
However, if you get and accept SVA approval as well at the same time...you will be able to have a fully acting throttle on a post December 2015 pedelec with it still considered a bureaucracy free pedelec, a bonus.
There is further explanation of the above para in the "Getting legal permission to have a fully acting 'Twist and Go' throttle on a pedelec" and the related "Grandfather Rights" section further up in the same post.
TBH, though, so long as you don't take the piss e.g. blatant speeding, I suspect you're more likely to get done for riding without lights (or pedal reflectors, if the police officer feels like being really picky), than you are for having an illegal throttle, given that it is theoretically possible for an e-bike so fitted to be perfectly within the law. (In much the same way, I suppose, that it is theoretically possible to park a car on the footway without actually committing the offence of driving it on the footway e.g. if you have access to a crane.)
* This strikes me as being a somewhat starry-eyed view of the home-brew e-bike market.