All I know is that other countries make it much more difficult, if not impossible to, register heavily modified cars - engine swaps, for example.
Based on this article by Dale Lomas, it doesn't seem as if there is much chance of registering a Japanese grey import with an engine swap.
http://www.bridgetogantry.com/2/index.php/projectrx81/projectrx8/420-how-i-put-my-uk-track-car-on-to-german-plates
How I put my UK track car on to German plates
...An example of a bad car to try and import would be a Mitsubishi Evo VIII FQ-something. This is actually a Japanese model imported to the UK and not available across Europe (it has no European type approval number). Another bad example would be ANY modified car. All modifications to a German car must be backed up by paperwork that simply doesn't exist for most UK parts, and single-part approvals for each eBay or home-made part get pricey really quickly...