I very much doubt the British government would be inclined to excessive cruelty. A large number of British politicians actually sympathised with the rebels. If the uprising ends relatively early, it might even become ensconced in public consciousness as a stepping stone towards greater freedom, rather like a mix of Peterloo and the Chartist movement - all terribly regrettable and when you get right down to it they were *right*, but you couldn't be having that kind of thing, really, very sad.
Certainly there'd be a romantic tint to it. We seem to inevitably soft-focus episodes of sectarian or political mass violence, and the English-speaking world has a flair for mythologising the losers. If Washington & Co get royal pardons, they'll be icons of an Englishman's freedom in years to come. If they are hanged or otherwise die, they'll at least have romantic canvasses at the Royal Academy come the 1840s.