There is none.
As QNH says the notams read:
OTH : FROM 03/01/17 15:17 TO PERM B0148/03 E)FREQ 123.45MHZ NOT TO BE USED AS AN AIR TO AIR COMM CHANNEL WITHIN
RANGE OF ANY VHF GROUND STATION IN UK FIRS
And I have it on good authority that 'they' are listening out for any violators.
It is legitimately used for cross oceanic traffic.
Some people seem to use the gliding or ballooning frequencies, so I have heard rumours.
But that would obviously not be legal either.
FD