Good luck on your search.
Work on at least 10% discount for your GTI. I think £20k is optimistic but, you have to start somewhere.
I managed 10% on the Pirelli Edition despite the dealer saying that they wouldnt discount. I also got a decent price on the part ex. Remember that they normally value your car first. So if you dont like what they are offering then say so - but remember not to tell them how much you have to spend until the final negotiating stage.
My bargaining worked like this
1. In my head I thought my car was worth £13.5k (2 year old GTI standard with lowish miles - depreciating at £3.5k each year from new and worth 50% of retail at 3 years).
2. Nowadays 10% should be possible unless the dealer has already sold plenty of the model. So list for Pirelli was £23k meaning I was looking at £21k or £7.5k to change.
3. £7.5k to change was in my head do not divulge this figure to the dealer. When salesman first came back out his cost to change was £9k! I told him politely to stop wasting my time and asked for the manager that he had been going to see to come out as well. Keep it amicable not nasty.
4. I again asked for the best price that they could give and if they didnt get near the figure in my head then I would be walking (or driving up to the next nearest dealer). This time the manager went straight to £8k.
5. Advised him he was incredibly close but, I still couldnt deal at that price. This is when you know that they want to do a deal and they know you do to! This is where taking things off like paint protection (managed to get mine still free!) come in and you can use to get those last few £Â£ off and your free tank of fuel and any extras like mats or roof rack.
Dont be afraid to get up if you dont like the deal and say you want to think some more. We nearly went for some lunch in between negotiations but, they were eager to deal and didnt want us to leave.
Finally dont forget that if you buy your Golf before 30 June get the 3 years servicing for £200 - dealer didnt mention this to me until I spotted it in a brochure on his desk. Although it is meant to be for cars regd before 30 June they let me have it anyway!