Can I hide my hire-purchase car to stop repossession?

No

A car or any other goods on hire-purchase belong to the lender until the hirer makes the final payment.^[1]

If the agreed payments are not made, the lender can re-take the car or apply for a court order for its possession.

Preventing the lender from access to the car may be an offence of contempt of court.

^{[1] Helby v. Matthews [1895] AC 471}