As above, tell tale signs would be :-
1. Original PDI stamp in the Service book, (if imported will be a dealer in Germany, Holland, or Denmark etc...)
2. Foreign language handbook & manuals;
3. Number plates may not show a VW UK dealer name (but an importer name such as Motorpoint);
4. Puncture repair kit instead of spacesaver spare wheel.
Having said that the only sure way is to find out through VW UK, or via a dealer.
How old is the GTi he's looking at ? cos if it's more than 3 years old it will be out of warranty regardless of where it came from.