As Maydup indicates, Teletext is now largely a thing of the past. ITV closed its version (Oracle) down about 30 years ago, at the end of December 1992, and offered no replacement. The BBC ended its Ceefax version when analogue broadcasting came to an end in 2012 but partly replaced it with its Red Button services. A proposal to end all Red Button services was put forward in 2019 but it was decided, instead, to continue it on a very limited basis:
https://www.bbc.co.uk/news/entertainment-arts-54360998
BBC Red Button services are, probably unsurprisingly, available by going to a BBC Channel and then pressing the red button on your remote control. (Note that the Index page still shows many services which no longer exist. For example, if you try to access the National Lottery results, you'll only see a message redirecting you to the lottery's website. There's actually very little content left there now).
If, by any chance, you've inadvertently referred to 'teletext' when you really meant 'subtitles', the button you need on your handset is the one marked 'CC' (= 'closed captions').