It's been uncovered for all to see:
|This can be found overlooking the railway on Holloway Road opposite Upper Holloway station|
BRITISH MATCHES FOR BRITISH HOMES
( BRYMAY )
I am not sure whether this is a permanent thing. I hope so.
Brymay is a conjunction of Bryant & May.
See here for more Brymay ghostsigns in London
Read about match-making here.