Certainly an interesting idea! However, coming from the UK I can tell you principled stands against overreaching unions do not always go as planned…

Besides the infinite administrative and legal challenges such a secession would impose - challenges which in many cases simply cannot be managed - there are also some basic, practical concerns too.

Texas is a land-locked State. Getting access to the coast would be a major concern for them, do they now need a passport to leave into neighbouring states? How will they export/import goods?

How are diplomatic disputes settled? What about concerns for national defence? What would a war look like if it broke out between the US and Texas? Europe has fortunately (perhaps not the right word…) been united by the fallout of WWII and the imposing threat of Russian invasion, to which end the UK and EU will continue to be strong allies. Given the history Separatist Texans and US Democrats have, could the same be said in this case?

This isn't to say that self-determination is a good principle to support, but simply saying "well let them vote and see what happens" ignores the implications of what would happen if they really voted 'Yes'.