Im making an educated guess on this one so may be a little way out? The likes of the Job, and Aisa for a start never had a UK certification when new. This was the problem the Bolkow 207 initially had in the UK I think? There were several kept on the D- reg for some time.
The other issue would be product support, i.e approved mods, maintenance schedule and spares. I guess there would be none for either type so the aircraft would have to have a LAA permit.LAA permits for factory built aircraft have been a problem when flying outside the UK but I think EASA is sorting that out? Remember almost all European permits are only issued to homebuilt aircraft.

So basically it is down to paperwork. For instance there are many European designs of microlight that are not UK certificated because of the cost, hopefully EASA will sort this out?