As the title says, what if the Second Barons War had ended in triumph for Montfort and the Baronial cause, with the provisions of Oxford ending up becoming something of a second, far more encompassing, Magna Carta?
As a more specific POD, after Henry III of England had fleed the field of Lewes in defeat and sought refuge in a priory, the sources we have described his armors as damaged by the fighting, showing that it was something of a narrow escape. Therefore, we can easily imagine Henry being captured by Simon's forces instead of simply having to negociate a settlement from a position of clear weakness like in OTL, significantly reinforcing the, already quite solid at this point, barronial hand.
As a more specific POD, after Henry III of England had fleed the field of Lewes in defeat and sought refuge in a priory, the sources we have described his armors as damaged by the fighting, showing that it was something of a narrow escape. Therefore, we can easily imagine Henry being captured by Simon's forces instead of simply having to negociate a settlement from a position of clear weakness like in OTL, significantly reinforcing the, already quite solid at this point, barronial hand.