Television broadcasting in Great Britain (and probably Northen Irland) started before W.W. 2 Since television wheren't cheap, not to many of them where sold.
After the begining of W.W. 2, television broadcasting was stoped and tv sets (that where radio sets) where sold cheaper, because they could only be used as radios. Those one buying them cheaper where lucky people, because in 1946 they could use them again and new made sets wheren't probably cheap.
Begining to 1946 and up around 1960 who could afford a tv. How many tv owners where in Great Britain and Northen Irland in 1955-1960?
What kind of programs where broadcasted?
Anyway, why Great Britain didn't aligned itself to the 625 standard? To many sets arelady sold when 625 becamed the standard?
Here you can see some old tv sets:
https://www.oldtechnology.net/images/iconoldtv.gif