I read somewhere that if he had reached London, he might have rallied support in the south of England, but I was also told that the English armies 'put the fear of God in him' and he fled. That didn't turn out so very well for his cause, or the highlands either.
Does anyone know the justification?
Charles had persuaded his Scottish council to invade England by claiming that he had the pledged support of France and English Jacobites. After invading England and reaching as far south as Derby in the Midlands, no significant English support had been forthcoming and the French were nowhere to be seen (in fact they were preparing an invasion force in Picardy but as news travelled slow they did not know this).
Charles' council therefore now believed that he had lied to them and were not enthusiastic about a march on London with just the forces they had. What clinched it was the intervention at a council meeting of Dudley Bradstreet, who claimed there was a third British army waiting for them at Northampton (between them and London). Bradstreet was a government spy and there was no such army but it ended any hope of Charles persuading his council to march south. Charles reluctantly agreed to retreat north back to Scotland but this was completely against his wish to march south and take London.
Frank McLynn claims that English Jacobites would never have come out in support of Charles whilst the government armies of the Duke of Cumberland and Marshal Wade were still at large and undefeated. He claims they were waiting for a Jacobite victory in England before joining Charles. They were not prepared to take risks.
Source: Frank McLynn, Charles Edward Stuart: A Tragedy in Many Acts (London: Routledge, 1988).