To get a T1 form, you need to have a guarantee with HMRC or use a customs broker who has one to raise the form for you. This is because the company that raises the T1 is responsible for the Duty and VAT of the consignment if the T1 is not closed off at the other side of the transit.
In Order to raise a T1, you also need to have an export customs clearance, as well as copies of the commercial documents for the load. They will then raise the T1 for collection at the port of export.