Okay, a longer answer now I have a proper keyboard in front of me rather than just my phone.
In the UK, a microlight aeroplane must meet a set of standards called BCAR Section S,which is quite a lengthy document with a lot of requirements. A couple of those requirements are for MTOW and stall speed.
A light aeroplane in the UK has an alternative set of standards, which because of a capacity to kill more people at once, not just through the number of seats, but the total inertia, are higher standards. The specific rules for any individual aeroplane will vary a bit, but in all likelihood will be substantially a document called CS.VLA.
To register, or more to the point to obtain either a PtF or a CofA in the higher category,requires somebody to prove to an appropriate authority that the aeroplane in it's entirety, meets the applicable regulations - at the new higher MTOW.
For a lot of foreign aeroplanes this is hard verging on the impossible, because their designers may not have used the safety factors required, or just won't provide the evidence that you need. I used to do this for a living, and I'd say that 60+% of projects that we worked on failed because we couldn't get the information we needed from their oversees (or occasionally even British) designers. It was bloody frustrating sometimes.
G