As Socrates points out, it's not at all implausible that Britain could hold on to New York and New England. My only point of concern is that, had they managed this, chances are they would have won the war altogether, re-absorbing all the colonies. Think about it: following a crushing defeat like that (the whole north occupied/blockaded/lost to the indepence movement), can the southern colonies keep fighting? Yes, for a while. But can they win? Not likely. Britain would be inclined to fight on, knowing that time is on the side of His Majesty...
But suppose a scenario where Britain holds the north, and lets the south go; this would mean lots of patriots in occupied New England. Just as loyalists fled (or were expelled from) the USA, patriots will probably leave New England--or the British will have to repeat something like the expulsion of the Acadians, except with the American patriots. They'll mostly end up in the *USA. That's going to affect demographics (both of the *USA and of this ATL British America), so keep that in mind.
Assuming this is done, I must say I don't think these territorial additions will prevent some sort of ATL Canada from being established. Perhaps it will be more confederal/decentralist, but there are major benefits to such a union, and trade-oriented New York and New England would realize this very soon.