@steveo: EN15194 applies to the construction of bicycles and not to kits.
But the law which defines the technical requirements an e-bike has to meet in order not to be classed as a motor vehicle is based on that standard, so it comes down to the same thing at the end of the day: https://www.gov.uk/electric-bike-rules.
This comprehensive post from an informed source on pedelecs.co.uk gives pretty much chapter and verse on the law relating to road legal e-bikes. It includes a section dedicated to the issue of kit motors, the key parts of which I reproduce here for completeness:
...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.
However, 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 [EAPC] requirements so is exempt from being a motor vehicle and is approved as a [EAPC].
It seems abundantly clear (at least, to anyone not trying to find a way to justify the illegal use of a non-compliant e-bike) that anything, whether manufactured or kit-built, that doesn't meet the EAPC standard is classed a motor vehicle and thus must be registered, taxed, insured and ridden as one in order to be used anywhere other than on private land with the permission of the landowner. The fact that some people "get away with it" doesn't of itself make it legal (see also driving offences such as speeding, using a mobile phone while driving etc etc which I imagine everyone on this forum would deprecate).
Note that the post also discusses twist-and-go throttles. These were legal (in the sense of not being specifically illegal) under the 1983 regulations, but were outlawed in the 2015 regulations. However, a bike manufactured or built to comply with the 1983 regulations before 1/1/2016 could continue to be used under "grandfather rights" so long as it remained unmodified BUT that also meant that the motor was limited to 200W as allowed under the 1983 regulations, rather than the 250W allowed from 2015. However, it is actually theoretically possible to get an otherwise compliant 250W EAPC with a twist-and-go throttle approved as a "250W Low Powered Moped", which would also not be classed as a motor vehicle, but again it's unclear whether anyone has ever tried this.