I dont know where you have been looking but there are loads of 24" TV's. Panasonic, Samsung, Sharp, Toshiba, and all the cheaper makes like Cello, Bush etc have them. I dont think you have been looking hard enough. Black shirts have plenty too in store. Sainsburys, Tesco larger stores have plenty.
for instance
Panasonic 24" 2017 model
http://www.panasonic.com/uk/consumer/televisions/HDTV/tx-24es500b.html
bit pricey though.
I have a TX-24C300B that I picked up a while ago for our lounge, its meant to be a 720p TV but it actually does display 1080p picture which I always find strange. Picture quality is pretty good. It even says in the display info its 1080p. There are plenty of £150-£225 24" TV's if you look ranging from budget to Smart TV.
Quick search on Tesco website reveals 8 different makes of 24".
Worst thing about 24" TV's are the speakers they are a bit rubbish.