Alternatively, without Cromwell, the Levellers could have actually expanded their influence, since Fairfax IOTL was kinda unwilling to stand in the way of Army politics, and Henry Ireton seems to be someone who could well make compromise without Cromwell. If it turns out to be a Leveller-wank TL, Charles would have lost his head, and the Commonwealth would actually have greater chance since many Leveller ideas would have been popular to common folks (e.g. abolishing debt imprisonment).
What were Pym's politics?
However, Pym is someone who could well recognize that Charles was a dead man by 1648 and that the NMA was a too significant force, and have sufficient stature to influence the Long Parliament to execute the King, which means, the Commonwealth of England could be formed without Pride's Purge.