All the venues will have been required to announce the show and put tickets on sale at the same time at the request of the promoter. All small venues guaranteed to sell out so the promoter wont have wanted any of the venues doing their own thing.It looks like it needed a tip off. I'm on the mailing lists and follow the venue and promoter on twitter but didn't get any notifications.
Perhaps Chrissie Hynde put the info out.
She loves playing small venues. I was lucky enough to see her perform in what was no more than a pub in Hertfordshire about 5 years ago.Bit of a coup getting her there