These are my thoughts just so I don't want to be seen as putting a downer on what Northern Ireland has achieved today, as it is amazing and a wheel in the right direction.
Where kits are concerned we are all in the same boat Gazwald, none of the laws make any mention of kits in the UK and Europe. All three laws, technical standards EN15194, the exemption from motor vehicle type approval 168/2013 and the usage on the road law EAPC 1983 as modified 2015, they all only refer to complete manufactured electric assist bikes, trikes and quad bikes.
So both individuals and suppliers operate in a legal vacuum in which all try to supply and use as closely conforming to the complete e-bike law as they can, trusting that is acceptable. That has always worked elsewhere 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 theoretical most legal way of dealing with a kit, although 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) Normally the purpose of this is to get an SVA (Single Vehicle Approval), entitling it for use as as type approved motor vehicle. However your aim 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 instead of SVA.
But as said, it's never been done and in any case there is no approved testing station in N.I., so you'd need to bring the machine to the mainland UK. But here there is only one testing station who would know what you were talking about, the one at Southampton!
That's because an arrangement has been made there for pedelecs with an added otherwise illegal throttle to be tested for L1e-A approval to make the throttle legal in the UK only under a DfT approval.
But to my knowledge they've only ever done this once.
In summary, legislators don't like kits, too many variables, so avoid their mention like the plague. Mainland Europe killed the kit car industry long ago and our laws have damaged it here too, so we are perhaps lucky that the lawyers don't pay attention to our little pedelec kit niche.
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