Great Britain Philatelic Society

I came across this cover with a nice machine slogan. The cover was being sent to Canada with a 1/- Wilding but required an additional 3d stamp which was placed lower left. It was not possible to cancel the 3d stamp with the machine slogan so a handstamp was applied. The handstamp used is a barred London cancel. Can anyone identify this mark? I couldn't find any reference to it.
Thanks,
Daniel