Yes, you can use your current car as a deposit. If it was previously financed and is fully paid off, you can part exchange it and use its value as a deposit for your new car. If your current car is still on finance, whether you can use its value for a new car depends on whether it is in positive or negative equity.
If your car is worth more than the current finance, it is in positive equity. This means you can trade it in, pay off the existing finance, and use the extra money as a deposit for your new car.
If your car is worth less than the finance amount, it is in negative equity. This means you would need to trade it in and pay the difference between its value and the finance amount from your own pocket.
For more information, check out our guide to part exchanging a car on finance.