Red tape abounds on cross border movements of cars other than short term (holiday, etc.) - in any country. This has to do with taxation, mafia-like market structures (protection), etc. Note that if you move a British car abroad for more than a year (at most) or even less in some cases then you are likely to be expected to register it abroad. Continuing beyond the (overseas) permitted time on British registration is likely to be somewhat risky, particularly if you are ever involved in an accident because insurance issues become very woolly. The same applies more or less exactly to vehicles brought to Britain, except then not only do the lights need to be changed but the car must read speed in miles to be legally registered - this is a hurdle which usually does not arise in reverse because (most) UK cars show both; it is a British nod to the majority.