You have to pay VAT and Import Duty on both the good and cost of shipping. I would ring the Customs and Excise commodity code section 01702 366077 and check the code for what you want. If you don't you can bet UPS etc won't look very hard (there's an awful lot of commodity codes) and will just charge a standard 10%. For the description "car parts (i.e. Superchargers/Turbos etc) for use on spark ignited petrol/diesel engine" commodity code is 8409910000. This equals 2.7% import duty. Give customs a ring, they're unhelpful, but you should get there in the end. (12/99)
Just spoken to C&E and got these commodity codes:
Description Commodity Code Import Tax
Car parts - body 8708299090 4.5%
Car parts - brake pads 8708319900 4.5%
Car parts - brake discs 8708399000 4.5%

Not great, but better than 10%. (2/00)
Finish line won't charge you VAT or duty - it's nothing to do with them, but you can ask if they can put the correct description of the items on the box under the "value for customs" bit, and include the commodity code. Customs will charge on entry to the UK, which your shipper pays and then charges you. Make sure they know the commodity code, or if they make you pay the full anount, then claim back by calling your local customs and excise people (2/00)