If you are flying into the UK – here are some useful links:
UK Border Agency page for Pilots
Details border completion and submission instructions for GA IFR and VFR pilots flying into the UK or leaving the UK: Detailed instructions
Handy Spreadsheet to complete your submission: Spreadsheet
PDF version: PDF Form
From the Border agency website:
Covers all International and Channel Islands traffic which travels to or from the UK but does not operate to a specific and published schedule.
An operator or pilot of a general aviation aircraft is legally required to provide notification of flights if they are making an international or Channel Islands journey to or from the UK, unless they are travelling directly from the UK to a destination in the European Union.
Even if operators and pilots are not required to provide notification of a flight, it is helpful if they do provide this notification.
To provide notification of flights, operators and pilots must complete a General Aviation Report (GAR) form.
The requirement to provide notification is made under: