well assuming Texas breaks off and establishes some type of real independence... While i expect warm relations with England and France to follow (as you said) i honestly think the Civil War will be the end.
UK and France will NOT go to war over Texas, for the same reasons that they didn't go to war over the South. Not enough in it for them to fight a war (that i think they'd lose anyway). Too much trade with the North, the possibly of dying British lads in Canada (who knows?) and Texas, dislike of slavery...and for what?
You may well be right and you could certainly make an arguement for it. The reason Texas wasn't accepted into the Union the first time they petitioned was over the slavery question. Adding another slave state at the time would be undesirable. Establishing Texan independence may quell the expansion of slavery in the US and allow it to slowly die off.
But our arguement was over Texan involvement in the Civil War. Thing of it is, Lincoln wasn't a fool. Strategiclly and politically declaring war on Texas would be suicide for the US.
Strategiclly, it would be another front and a larger foe. Texas fielded 70,000 men for the South in OTL. While only roughly half that number would probably be in an actual field army, it's a force in it's own and would make the war that much harder. Especially since Albert Sibley Johnson would presumably be the general-in-chief of the Texan army.
In OTL Texas was never invaded successfully. It's an isolated area in this time and the logisitics make it even more difficult for an invading army. Plus the possibility of the Texans linking up with the Confederacy and reinforcing their Trans-Mississippi region can't be ignored.
Politically? Declaring war on a country in which you have no territorial claims on or legitimate quarrel? Short of catching the Texans red-handed crossing the Lousiana or Arkansas border how much are they going to be able to prove? Even if they do, all Austin has to do is deny any knowledge. Smugglers are common in all wars.
Regardless of the outcome of the war, Texas stands to gain a fortune running guns in and cotton out of the South. As the South would be even weaker ITTL than OTL even with an open port in essence in the case of Galveston, Texas would probably still be too insignifigant for the Union to get hot and bothered about.
In the case of a declaration of war on Texas at the very least France would be dying to get involved. This is of course presuming they still install Maximillian as Emperor of Mexico ITTL. It was a forgone conclusion that the US would overthrow this regime as soon as the Civil War winds down, which did happen in this timeline.
The wildcard in this scenario, actually this whole topic, is California and the Mexican-American war. Is there a second Texan-Mexican war? Does California achieve independence? What effect does the gold rush have on this? Maybe even the Morons achieve independence?