As far as I know it is triggered at certain (not sure if all) the intelligent mail processor centres in the UK when a BT postcode is recognised.
A picture will be added if anybody in Northern Ireland sends us one!
Update 15 May:
Thanks to John & Patrick we now have confirmation that there were indeed 11 different postmarks in use for all the team members. However, until examples are found, nobody will know whether any of them was actually used and from which sorting offices.
This is the list:
S (Sheffield) Banks
WD (Watford) Barnes
BT (Belfast) Best
SA (Swansea) Charles
NE63 (Ashington) Charlton
E12 (Manor Park/Redbridge) Greaves
DN3 (Doncaster area) Keegan
AB10-13, 15, 16, 21-25 (Aberdeen area) Law
EH (Edinburgh) Mackay
IG11 (Barking) Moore
DH2,3 (Chester-le-Street) Robson
As before - please send your images of these if you find any!
Thanks to Kevin McShane, Hon Secretary North of Ireland Philatelic Society, Belfast, I can now show some examples of the George Best slogan. Kevin writes: "About one third of my mail was cancelled with the George Best Slogan with most of the mail arriving with the usual orange coding across the stamp and no slogan. Northern Ireland really only has one main mail centre at Mallusk, Belfast. The Prostrate Cancer slogan was also in use on the 13th. I got a few covers sent from Liverpool to BT1 however these arrived with a standard Warrington Mail Centre cancel and no reference to the football slogan."
[Details of the stamps, booklets etc including retail cylinder book are in the main post.]