VWFS will not offer you terms based on less than 70% of the RRP of your car being financed. If you want a £27k GTI on VWFS PCP, you will have to have £18900 on finance.
If you get 12% discount (£3240) and the £1250 deposit contribution on a £27k GTI then you'll only be able to add in £3610 of your own money to get to your £8100 "deposit".
So you could stick just over half of your p/x money in the bank, along with all of your shares money, and be paying around £280 a month over 36 months. Your money will earning £50pm in a Santander 123 current account (3% on up to £20k balance).
Alternatively, you have the means to buy it outright - 12% discount, £1250 deposit (take up the PCP and then settle within 14 days cooling off period for financial products) and you'll have about £2500 change from your £18k + £7k p/x. As far as the mortgage company is concerned when assessing affordability, that's £280 a month NOT committed to going out of your account.
Realistically your GTI will be worth £14k in p/x at 3 years old, so it will have lost £8500 vs the £22500 that you paid after discount and deposit contribution. Over 36 months, you'd need to put in about £230 a month to cover that depreciation (£236 a month on the assumption it makes no interest). So buy outright and the car will cost you £236 a month in depreciation, buy on PCP at £280 a month and make £50 a month in interest, but you will have already lost £3610 of your own money in the initial deposit, but the car will have around £1500 p/x equity.
Taking all that into account, you will save about £2100 on a £27k GTI by buying outright vs PCP over 3 years. Cash buys are also less hassle to sell privately at any point in time if you get itchy feet.