I once had the job of driving around North Wales for the day trying to identify potential landing strips for small aircraft infiltration.
Let's just say I logged quite a few.
I can quite believe it, on a couple of occasions while I was at Ty-Croes in the first half of the 90's, some form of surveillance operation was done from the T94 based there which involved unknown people coming on site and the Techies being barred from the ops cabin. I suspect it was something to do with the IRA and gun running, but we were told not to ask the scopies what it was all about.
The radar coverage of the UK coastline has never been really good since the Type 54 CHEL stations were shut down in the late 1950s and since then, the coverage towards France at low level has been non existent. The AD system from 1960 through to 1968 was aimed at dealing with reconnaissance threats to the V-Force (not attacks on as they would have been stand off or ballistic in nature). After that, Linesman which tried to interrogate the AD and high level ATC system (and failed), Followed by UKADGE, IUKADGE and UMCP which has just been replaced. Since 9/11 there has been a major improvement in low level coverage with the introduction of a number of military ATC radars into a networked feed into the CRC's known as AIRP.
https://www.sell2wales.gov.wales/sea...x?ID=AUG150943
However having the intelligence is useless if there is no way to catch them in the act and they are really only interested in somebody who is not flying to a flight plan.