In the not too distant past (in the 800 year history of the Royals) Mr. Fagan would have been dispatched on the spot, when he was finally discovered. Does not say how he was discovered, but no doubt the Queen called in the guards, silent alarm? And some outdoor guard paid for allowing this guy in— a 20 foot wall- that is some kind of climb, and then up the rain pipe.
Odd site to recall the tale— rick steves the uh.. oh forget it.
IIRC the Queen did use a silent alarm but got no response. A chambermaid eventually entered the room and asked who this person was to the Queen. The look from the Queen was enough to get the maid to alert security whereupon the incident ended.