I'm pretty sure that you aren't allowed to stow items in vehicles when they're being shipped, but the reason escapes me just now. ?Perhaps it's a safety measure in case they need to get into the car in an emergency or possibly MAF don't want to unload a car full of stuff so they can do their inspection.

Don't know if you'll find this link about importing vehicles useful at all. ?I can't see mention of not filling the car on here, but I'm sure I've come across it somewhere. ?Perhaps someone can confirm or put me right if I'm wrong.

Aah! Here's something from Customs . Not so much a case of not being allowed, just that it's not recommended.