The owner writes a bill of sale to the new owner and sends it to the shipping agent along with the title, he then liases with the british shipping agent, it all gets sent to british customs who assess what duties should be paid. However, if a car is over 30 yrs old, you can apply for a lower duty rate which is a flat rate 5% value added tax.
If the car is newer, you pay 10% import duty on the car and the shipping, then you pay 20% on the car, duty and the import tax, so it compounds.
Dodge Dart GTS, 440ci, RPM heads, .590 cam, NA
1.41 60FT, 6.43 at 106 mph, 10.16 at 131.62 mph