Did this advice come from an IP or a member of their senior staff, or someone sat in a call-centre? I am alarmed at the fact that you have been given this advice and so would the IPs regulators and the OFT. this smacks of trying to manipulate an IVA to work at all cost - and yous should only hand the car back to Santander with the absolute agreement of all the remaining creditors.
Hi
If the car is of high value then you may be asked to sell it and downsize with the balance introduced into your IVA so that all your creditors receive a fair share.
Regards