Who said they should've fought everyone? I said they should've discouraged the USA from bullying Mexico, with French and probably Spanish help. That's hardly “attacking everyone”. Hell, they would be the “good guys”.
No you didn’t. You said:
“Of course, they should've supported AH and Mexico before the Germans and the Americans rise.”
France was barely mentioned as an afterthought until several posts later. And was about to have its government overthrown. Spain wasn’t mentioned, and was in the middle of the Second Carlist War regardless.
But here’s the thing, your fundamental argument is that Britain doesn’t want to ever be a junior partner in anything, and in 1848 Britain being weaker than the pitiful US was LAUGHABLE. The Americans barely had a military, and their economy was afraction of Britain’s juggernaut, they didn’t even have as many PEOPLE.
If Britain has decided that smashing the United States is that important they are going to see the same threat in France. And Austria. And Prussia. And Russia. And Sweden. And every other power. Including Mexico come to that.
France invaded Mexico for absolutely no reason to install a puppet monarch, and they didn't got a coalition in their asses. Why would Britain be the target of one by defending a minor power?
Britain isn’t defending a minor power. They are actively trying to cut down a smaller power by your own admission. One who is, from a legal standpoint, fighting a defensive war since Mexico attacked the US army first.
And I would note that the belligerent idiocy of Nappy left him alone and friendless when he went looking for help against the Prussians. He’d alienated or pissed off every potential ally, and no one cared when he jumped into his own grave.