Don’t think you’ll find one for that budget
I’ve just had a quick look on the Auto Trader website and using the search criteria of £3,500 max budget and searching the whole of the UK, I wasn’t able to find any mk6 Golf GTI’s. There were a couple of mk5 2008 registered GTI‘s - these were priced at £2,995 and £3,490. Increasing the budget to £5,000, and there was one early (2010 registered) mk6 GTI at £4,995. These were all trade sales, so if there’s anyone selling one privately, it might be a bit cheaper.
@Fraz, if you’re not able to increase your budget just now, then it’d probably be worth waiting a bit longer until you’re in a position to increase your budget.
Good luck with your search.