Details of the failure are important. As stated above, the French Revolution may be affected, especially if France hasn't spent a fortune (for no gain) on the AR.
If we simply let the rest of the world march on without butterflies:
Without a handy US buyer, and the unlikelihood that France would sell to enemy Britain, France may simply hand the property back to Spain. Of course, in the failed revolution, it is Britain who will recognize the importance of New Orleans and seize it from enemy Spain. Ultimately, Britain is going to force France off the continent. the only question is whether Britain gets it, or Spain gets it, or whether they split it. I can easily see Britain keeping NO. at the congress of Vienna they had no problem keeping whatever they thought useful for themselves.
Popular belief has Britain freezing white population at the proclamation line of 1763, but it is highly likely that an expansion is going to take place. Britain's policy toward the natives is important to the scenario. Will it be as outright hostile and one sided as the US policy, or will it grant the natives a greater piece of the pie? In the end, the natives likely get screwed, but do they get screwed as badly as OTL? I don't know my Canadian history enough (shame on me), but several here have stated in the past that Canadian treatment of natives is not cause for optimism of British treatment of natives in a British colonies TL. Any way you look at it, the expansion scenario is going to be different. knee jerk reaction is that it's likely to be slower. That bodes better for New Spain/Mexico and pressures on it.
How Britain manages to hold on to, or eventually lose, the colonies is also important. For arguments sake, lets say the colonies get some home rule, gradually increasing. The wilderness territories gradually end up as new colonies. there is no one to stop this expansion except the natives, the spanish, or the russians. Count out the natives. The russians, unless they amp up the OTL efforts, aren't really a serious threat below Alaska. People overstate their presence in California.
Spain claimed the entire Pacific coastline. Britain mostly laughed at that. Expect here that Spain/Mexico gets California, and Britain ends up with Oregon/Washington/British Columbia. I also expect that at some point, Alaska ends up in British hands. I don't expect Britain to violate Spanish claims on Cali. That leaves the Louisiana Purchase region in question. If Britain has wrested New Orleans from Spain/France, most of the northern parts of it are likely to follow. My thought is that when Britain takes NO, they simply take all of it and give Spain a generous borderline.