This isn't simply a outstation problem. I've seen Speedy Boarding go wrong at LGW South, too. On one occasion, as soon as the gate number appeared I made my way to the gate only to find that SBs had come and gone and that group A was boarding! (I had to ask those milling around, too, as there was no indication which groups had been called to board. Why can't this be put on the monitor(s) at the gate?)
I get the impression that when it's important to get the aircraft away quickly, the priority system is sometimes abandoned completely. This has happened to me on a couple of occasions - once each at LGW and LTN.
These flights were at least a year ago (I don't use easyJet much, preferring Ryanair) so things might have changed.