What would people here do? The mega safe option is to just withdraw all the investments and use that for a car. However, i believe there's an opportunity cost there and would hope my investments return more than the interest payable, although obviously it's a risk as we all know anything could happen in the next 12-15 months!
If you fancy the gamble, you needn't be all in. Withdraw part and use a loan for the remainder?
Personally if I had a one year timeframe I would take the 15k out of the stock market and put it into a one year bond paying a guaranteed 4% or so.
1 year fixed savings rates are about 4.5% at the moment.
Spread some or all of the cost of the car on the a interest free credit card and I'm sure there's a net gain in there somewhere.