The General Aviation Report (GAR) form is a form you fill in and fax to the appropriate UK authorities an appropriate number of hours in advance (details on the back page of the form). This basically allows you to fly into and out of the UK direct via any UK airfield - it does not have to be a customs airfield. Obviously there are some limitations, but it's very convenient for private flying.
It's only relevant for the UK side of things though. For the mainland Europe side of things you need to fly via a customs airfield such as Texel.
Download here:
HM Revenue & Customs