I once imported a car and was advised to follow this procedure:
1) Get insurance (giving chassic number since car is unregistered)
2) Do any work necessary for MOT (can drive it to and from a garage if necessary).
3) Book MOT and Drive it to MOT station (On the way there I passed three police cars while driving with no numberplates or tax disc and they didn't give me a second look

)
4) Go to DVLA office and fill out registration paperwork.
5) Tax disc and Log book arrive by post
6) Buy and fit number plates.
I originally played it safe and trailered the car from the docks to my house, because I wasn't sure whether it was in a roadworthy condition. Since it was roadworthy, I *think* it would have been legal for me to drive it home unregistered. Plenty of other people were doing it.
As I understand it: It is legal so long as you are only doing what is necessary to legalise the car.