I think that you will need to write or e mail Porsche in Holland and request a Certificate of Conformity.If you get a full Certificate , it is a doddle from there.I suspect you will get a Certificate of Partial Conformity. The Partial Certificate of Conformity outlines differences between Dutch spec cars and UK based cars.Porsche at Reading will not have any details of the car. There is often a fee for a Certificate.
Once you learn what the non conforming items are you will need to change them and submit the paperwork and an application to VOSA , and you will probably have to take the vehicle for inspection at VOSA. Non Comforming items are generally quite simple such as relocating the rear fog light to the opposite side of the car and changing the headlights to dip correctly for use in the UK.
Good luck and I trust you do not encounter too many problems on route.
I recently went through the fun of Registering a LHD Mitsubishi Pajero in France that was UK registered but since it was originally manufactured for the UAE it did not have a Certificate of conformity at all ! I could write a book on what I had to do to get it registered.
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