Edgehill would probably be the best choice, as it would allow the King to lay siege to London and probably end the war before it fully begins. As for Scotland and Ireland, yes they were issues, but more or less second compared to England. After all, as Cromwell proved, once you control England the other two easily fall. I can see a rap up campaign in England for maybe a few more months, then the King could turn to Scotland and Ireland. Of course, on the other hand, if Charles wins to quickly it could lead to more tensions, with people thinking its a "false victory" or something. But if he takes London, Charles would have to arrest the most vocal of opposition MPs and Lords and no doubt put a few to death as well. After that, get a new Parliament (maybe fix the charters to allow more favorable MPs, like Charles II did in the 1680s) and get them to give some taxation powers to the King, de facto legalizing his actions in the '30s.
As to Scotland, the King MUST invade, not gather another army and launch a few skirmishes. Unless he's able to occupy Edinburgh and Glasgow, and dissolve the Kirk he'll be in the same position as the late 1630s.
In truth Ireland might be the easiest to deal with, as Charles himself was pro Catholic (ie giving them rights, not restoring England to Rome) and should be able to win over most of the Confederates with concessions, religious rights and land restorations.
Sadly, I think a lasting success in regards to Charles's policies would require a more forceful, world based Sovereign, similar to his son.