If you buy a vehicle outside of UK and bring it back theres duty and VAT (=30% of the assessed value of the vehicle?) to pay although you shouldnt have to pay VAT twice ie. claim back in the country of purchase or offset it in the UK. I believe theres no exceptions to this unless youre moving home from another country to the UK and bring the car with you as part of your belongings.
However if YOU owned the car in the UK, took it abroad and registered it there but are now bringing it back thats different and should allow you to avoid these charges.
Saying all this I dont know how the law stands with regard to the Channel Isles...