Indeed, thoughout Europe including here in the uk they have no legal status. Enforcement is possible however, by taking them off the road but the authorities have no intention of doing that. They are well aware of their presence but take no notice while their users largely comply with the law for manufactured pedelecs.Isn't it also the case where all home built / modified are technically illegal regardless of 250w 15mph making enforcement largely unenforceable?
Overall it's not a big enough issue to be bothered with and drafting a law for home constructions would be a complex nightmare.
They don't have to be illegal though, since I can tell you the legal way of dealing with a kit, athough no-one has ever done it:
1) Buy and fit the motor kit.
2) Make an appointment at an approved vehicle testing station, paying the £55 test fee.
3) Most often the purpose of this is to get an SVA (Single Vehicle Approval), entitling it for use as a type approved motor vehicle. However your intention will be to get the inspector to agree that it meets the pedelec requirements so is exempt from being a motor vehicle and is approved as a pedelec.
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. I repeat though, no-one to my knowedge has ever done this to create a kit pedelec so it's never been necessary, but it is the DfT specified correct way when creating any motorised vehicle from more than one vehicle or from parts.
.