I'm not a Texan but I lived there for twenty years.
People's reactions in the UK: lots of the tired old stereotypes, not always meant in malice, but it can get old after a few years/months/weeks....
Upon learning you are a Texan you may be asked:
Where's your hat?
Did you live on a ranch like in the TV show "Dallas"?
Is it all desert and tumbleweeds?
Where's your Texas accent?
So you have no lakes or trees because it's so dry?
Are you stinking rich like the Ewings? (Because everyone in Texas is rich...)

On the less lighthearted side, I will say I have encountered
some UK people who, upon hearing I lived in Texas, truly and seriously believed that this meant I must ONLY be a racist, a bigot, anti-gay, and a staunch Republican, because aren't all those things what 100% of people in Texas are? (I happen to be the opposite of all those things.)
Even as a non-native, I find those discussions hard not to feel pissed off by, not least for the fact that these people were/are incredibly hard to convince otherwise, even though they were ignorant and had never personally been there. They especially wouldn't believe the picture I painted of the liberal, artsy, laid back and tolerant culture/circles I ran in in Austin, for example.
But then again, it's not just folks in the UK -- an NYC friend of mine, cultured, cool, educated and open minded, also did not believe that Texas was not the wasteland she believed it to be -- so you get this from Americans too, as you probably know.
Just to say, lots of people will have gentle fun with you about your specifically Texan heritage, and some will even have a positive opinion and fascination; but there will also be some individuals who are negative and hold erroneous and offensive stereotypes they have consumed wholesale from sources other than personal firsthand experience; but then they would be bringing that ignorance to a conversation with someone from New Jersey, California, or ...Pakistan. Have fun with the nice assumptions, but I guess just roll with the stupid or aggressive ones. (I don't always manage so well.

)