You need a classification code for your goods to enable you to fill out your customs documentation correctly and pay the correct duties and VAT. You can find the classification code by using the Tariff, but if you're not sure that you have the correct code, you can apply for Binding Tariff Information (BTI).
BTI is a written and legally binding decision on a classification code from HM Revenue & Customs (HMRC) that applies for up to six years from the date of issue. It can help you with particular types of export and obeying rules of origin.
An application form (C103) is available from the Tariff Classification Service.
You can also apply electronically for a BTI decision.
Some goods are restricted or prohibited for import and export and it is important that items are checked before they are shipped. Volume one of the Tariff contains a chapter on banned and restricted goods. Read more about sanctions, embargoes and restrictions.
For a general overview of the documents you might need for importing and exporting, see international trade paperwork: the basics.