It reads, 20 Great Queen Street, Covent Garden.
Cool huh?!
It appeals to me on many levels; geometry, typography and graphic design.
I am not sure who installed it/ how long it's been there.
It is set into the very front of the metal strip that runs across the doorway of No.20, to the left of Walker Slater menswear shop (at No.19) shown here from Google Streeview: