If the car comes up as an import in a history check (ie HPI report) it'll be valued as an imported vehicle. This could be marked as an import or other signs are the transmission type states "unknown", Co2 "unknown", body type "unknown" and sometimes engine number "unknown" Depending on how old the car is, condition and miles it will effect the value.
The perception of customers that come and buy from me is that the first owner saved thousands and thousands when they bought it (not the case but when your asking people to pay money for something their opinion as all that matters) so they are not prepared to pay what a legit UK car is worth. Not that it'll effect a MK5 but different market places have different warranties so on newer cars a main dealer would not re sell it so it'll go to auction. There could also be specification differences depending on where it started life. When I was at BMW a UK Msport had climate control and PDC, the "same spec" import had neither.
If you've got a young, mint and low mileage MK5 it will probably effect the value on p/x but if it's an older, shed with high miles probably not.
PM me the details and I'll look into it if you want?