Not sure if the car companies are getting wise to this now. It's worth doing and requesting a settlement figure immediately - you'll be paying £4 or £5 a day interest within the 14 day finance "cooling off period". If you Buy a £27k GTI with a £5k deposit, your settlement will either be £22000 or £22750, if it's the former then it's a free £750 if you have the funds to buy it outright, if it's the latter, you'll have to stay with them longer to get that "free" £750 and start paying proper interest. If your PCP ends up at around £320-350 a month over 3 years then around 1/3 of your monthly payment on average is going in interest.
In the early days, around 40% of your monthly payment is interest and once you're beyond the 14 day cooling off period, you will pay 2 months penalty interest for an early settlement. Going a month into your settlement to chase £750 (if you will get it after a month) will cost you £390 - £420, if you have to go 3 months in and in effect pay 5 months interest, to chase £750, it will have cost you £640 - £700.
Still worth a punt either way. I'm sure the supplying dealership don't get their commission unless you've been in your agreement for 3 months before cancelling.
For those of us tempted to do the 14 day cancellation just to snag some cheap servicing (GTD/R), I think it would be harder for them to screw you - there is no cash incentive, they'll either let you have the service pack at the agreed price that you were invoiced for, or they won't (will then cancel your service pack and your settlement will be £199 or £149 less than it would've otherwise been) and you'll be £4 a day interest down until you settle.