Do it this way then,
Keep the Golf.
Bosh £3.5k payment/deposit for the Rocoo on a credit card, they're always offering 6-9month interest free if you balance transfer. Stick the rest on a loan, that gets you the Rocco. Not having a car to p/x may give you a better hand to play with for some cash back or a better deal from the stealers.
That gives you 6-9 months breathing space to sell the Golf, which shouldn't be a problem, the sale price from that pays off the credit card. It's not a lack of cash problem but more of a cash flow problem.
The monthly payment difference from not having a £20k loan but a £16.5k loan you can put to one side and help pay off the card bill later down the line, if you get your preferred sale price for the Golf then you got a little chunk of cash spare to go on holiday.