Intervene in what way? The American military of this period was pretty tiny, so direct military intervention would be a bad idea (not enough to make a difference, but enough to seriously annoy the British). Covert support (in the form of supplying arms etc.) would be more plausible, although I'm not sure it would be enough for the rebels to win.
Down the line, if the US Civil War breaks out ITTL, you can probably expect the British to openly and actively support the Confederacy, as a way of getting revenge for Canada/neutering the big threatening state that keeps trying to invade British territory.