My experience is with military radios, but I assume civil radios also continuously monitor guard (in addition to the active frequency) unless guard is toggled off. Which does happen, because of clutter.
The civilian radios I've used don't monitor guard unless you dedicate a separate receiver to the task. Most of the planes I've flown in recent years have three VHF transceivers with one dedicated to VHF ACARS. There is also HF and SATCOM mixed in the selector panel. A lot of buttons with multiple opportunities to mess up.
Post 9-11 we really are more aware of the requirement to monitor guard in the U.S. domestic environment. It's always been a requirement on international flights as far as I know.
You have the 'guard police' who come up on the frequency to scold perceived violators of their radio discipline policy. 'You're on guard!' is their eternal cry, even when someone is legitimately trying to seek emergency assistance.
Every night over the central U.S. the guard police catch hapless FedEx crewmembers calling MEM ops on 121.5. Maybe it has something to do with their standard radio setup during preflight.
And, on occasion, you get a magnificent PA announcement on guard by a Skygod from a formerly bankrupt Once Great Airline (e.g. AA, DL, UA).
I've certainly been asked to try to contact an aircraft that missed a frequency change. And, most ACARS installations can sound a chime in the cockpit if the company sends a wakeup message.
Wouldn't BA 108 normally have CPDLC comms with the LHCC controllers prior to entering the airspace?