Customs clearance will be required both in the US and in the UK. It takes about a week for US customs to process the title of the vehicle. Most shipping companies will handle the customs clearance at the US end for you.
In general, this means that you must have clear title of the vehicle before you attempt to ship it. That means that your vehicle should be fully paid for.
In order to avoid duty (10% for cars) and tax (Value Added Tax - VAT - of 17.5%) you must have owned the car for 6 months or more in the US and keep it for 1 year after you arrive in the UK.
To prove that you have used your vehicle outside the EC you should bring with you any papers you have, for example:
1. Foreign registration papers;
2. Police certificate of registration; or
3. Insurance policy
You must also provide proof that you have possessed it outside the EC for at least 6 months.
The duty is based on 10% of the 'landed value' of the vehicle. This means what you paid, plus the shipping costs including insurance. And they will ask for evidence. The VAT is 17.5% of the sum of the same costs plus the duty paid. And you'll need paperwork to prove each cost.
Before you say goodbye to your car at the US end it is advisable to complete all the customs forms for the UK end. It's easier and more relaxed this way - though it is possible to complete them when the car arrives in the UK.
You will need form C104A which enables you to clear UK customs.
Also customs will ask you to complete a form C&E386 or C&E388 plus the Dept of Transport (DOT) Notice PI1 and leaflet V277 which sets out the legal requirements that you must satisfy before registering the vehicle.