Did it really benefit Britain to conquer the US after decades of independence, though? And Yeah, theoretically Mexico could hold onto Texas. As for an earlier or more successful secession, where exactly? New England?
I'm not talking of conquest of the entire nation as that's likely to be a disaster. However for instance.
a) 1812 conflict - could have seen a New England Confederation if a longer war had lead to the unrest in the region at the war to become an independent state. Also that Britain regains control of much of the 'Old NW and possibly later say the western border is set at the 45th parallel rather than the 49th?
One other knock on effect of this, given that the US would lose New England and also a number of future states west of Ohio, all of which would be 'free' states is that the rump US would be dominated by slave states - which could mean a later secession bid could end up being by northern free states seeking to oppose slavery being imposed on them.
It could be argued either way as to how a larger and more powerful Canada and a weaker but possibly still hostile US would benefit or hurt the UK but a lot would depend on the details.
b) By an earlier secession I was thinking IIRC an earlier attempt by the south to break away. With markedly less industrial development and railways especially the suppression of such a rebellion would probably be markedly harder. However again you could see something in the west - such as resulting from the
Aaron Burr conspiracy - assuming that this actually existed or in the right conditions a New England/Federalist break-away although I think this would have required support from a foreign power to protect, most likely the UK - although as mentioned in the link for the Burr conspiracy Britain wasn't interested in supporting any secession, at least until the US attack in 1812.
c) I think a Mexico that could become more stable and liberal rather than being wracked by dictatorships, militarism and corruption then it would be significantly more successful. For instance if American settlers were accepted into Texas and then sought to establish an independent state their far less likely to get the support of local Latino groups who were alienated by the Santa Anna regime, as were other groups in northern Mexico. If Mexico could avoid the OTL conflicts with the US or win enough that it avoided major land losses then a Mexico that includes Texas and California and points in between could become a significant power and the US would similarly be markedly weaker.