My understanding is that you can sell the car, but you would need to find alternative finance to cover any shortfall (ie a personal unsecured loan from elsewhere). Not sure if this is a viable option (esp. as you indicate you are getting late fees, which will no doubt be affecting your credit rating).
Another option is to potentially prioritise who gets paid first - it might be that you can let another bill be paid late without incurring such high penalties.
You say you are usually only a few days late, so it sounds like cashflow is the problem - Do you have somebody you could potentially borrow a months payment from, to get you ahead of the payments. You could agree to pay it back at the end of the agreement (if the friend/relative you have borrowed from can wait that long), or possibly pay it back in small amounts each week/month which are managable for you. This could be done via an Overdraft facility, but make sure you check the APR (and annual fees) of any Overdraft facility to ensure you dont end up paying more for it on your overdraft than you currently are in late payments.
You dont say why you are late - I have assumed above that it is a financial problem, but it could be that you just forget to send the money - in which case, setting up a standing order to send the money over on the correct day would resolve this.
Hope one of these ideas is of some use.
Fiamold aka Dave