Alot of this depends on which model and who your trying to sell it to..
P/X to a main dealer A large number won't accept it as a P/X or will offer you several thousand pounds less than the UK car would fetch. As they are trying to discourage people importing their own cars. You'll however notice a total absance of this attitude when it comes to servicing.
Private sale .. This depends on how desirable the model is. People will be put off by the fact that it's an import despite the fact that it maybe identical. On hard to get models (M - series mainly) it's less of an issue as there a fewer cars around. However on the more common mainstream models you'll have to discount the car to generate the same level of interest.
From my experiance of buying and selling imports i'd say normally they will be about 2 - 3 K cheaper than the UK cars although thats not set in stone i've seen imported 330's sell fast for uk car money. Just depends on who's looking to buy..