Customs and Documentation
Customs is a government agency responsible for regulating items entering a country. All items being sent to and from a country must clear customs first.
As an eBay seller, you'll need to provide the correct documentation so that your item won't be held indefinitely by customs. Your carrier can easily arrange for customs clearance before you post your items.
You may need to pay additional costs if items have to be returned because they don't clear customs. Also, goods can be held up if your carrier does not arrange for a customs broker in advance. (Most carriers provide this service automatically).
Full details of British and European customs issues can be found on HM Customs and Excise website