Pursebop- i found it on my last quick trip to London! I saw it on two seperate counters in Harrods (in two sizes medium and mini) in Selfridges and I think I saw it on Sloane st as well. (I went there for something else but I am pretty sure I clocked it when I room scanned!!)
I had the choice between this size or mini and went for the medium. I asked about other sizes and they had not yet seen it in another size. All shops had the bags on display and fresh ones in the back

Harrods first floor & Selfridges also had rose gold (darker) metallic reissues from summer season but not on display which were to die for!
Another poster found their mini in London.. so it seems to be that metallics have landed in London!