GAR'S ETC.

Let us make a couple of assumptions.
A) No one on board the aircraft is a non EU citizen.
B) It is not intended to take the aircraft outside the EU borders.

Areas of operation

A--Europe excluding the Common Travel Area (CTA)( Channel Islands,Isle of Man, Ireland including the North)
B--UK excluding the  CTA

Who to advise

Border Force (situated in the area of departure or arrival)
A--Special Branch (as above)
B--Customs (national coordination unit(NCU))

What is required

A--UK to Europe NO NOTICE REQUIRED
B--Europe to UK NOTICE REQUIRED 4 HRS
You can depart from/to any airfield/strip in the UK but only to a customs airfield in Europe

UK to/from CTA

12 hours notice required--- from from any airfield/strip both in UK and CTA-(note in southern Ireland a GAR should be sent to Irish customs, generally they are not interested unless you intend to pick up duty free fuel on leaving Ireland)

GARS filing

Can be filed to each individual unit or alternately via SKY DEMON, ONLINE GAR or free via AOPA GAR(requires registration)

ADDRESSES

The three above are obvious
From Haverfordwest (EGFE) 
Special branch/border force—dpp.portsinfo@dyfed-powys.pnn.police.uk   ph 01438871968
Customs—ncu@hmrc.gsi.gov.uk
Irish customs—kdeforce@revenue.ie

There is at least one out re GARS 
Should you be unable to file a GAR for return to the UK you can do so in retrospect at a UK customs airfield which may not have been your intended destination and you maybe delayed