I understand the question and have no desire to piss rip. I guess the answer depends on your ability to pay the monthly payments but assuming you can without concern then yes, I'd put the £16k in two ISAs where it'll be gaining interest on the whole amount from today. That £16k will grow over two years however it would lose at least half over the same period if you keep the car.
If you didn't sell it and started paying £400 a month into your ISAs then it would gain significantly less interest obviously.
So in your shoes, if I could afford the payments, I'd ditch your current car and get the money working for you. Oh, you also get a new car out of it - bonus.
Hope this helps.
David