If you buy the car, the opportunity cost is £12,000 and a motorbike.
If you buy the motorbike and have some money, the opportunity cost is one car.
Economists tend to measure opportunity cost in terms of money for simplicity's sake. However value isn't always monetary.
Another example would be if two women that you fancied, Sarah and Alice, asked you out and you said yes to Sarah (assuming monogamy), going out with Alice is your opportunity cost.