With regard to Hymie & Eddies posts, if there's no exemption (as in my post) then the '6-month rule' only applies while the person is not normally resident in the UK.
If, for example, someone comes here from France or Poland and sets up home here (selling their overseas home, or allowing the tenancy to end, and moving all of their family and possessions into their UK home) they must immediately register and tax their car as a UK vehicle, even if it's only a week after their arrival. (i.e. the 6-month rule ceases to apply at the point where they become normally resident in this country, so going back briefly to their home country or taking a day trip abroad WON'T restart the clock)