Here is the OFCOM document on which channels are required to have subtitles in 2018.
https://www.ofcom.org.uk/__data/asse...es-in-2018.pdf
See page 5 on that PDF (that's the marked Page 5 of the document, not the 5th PDF page by the way). There are 3 levels of requirements.
Under Ofcom's broadcasting code (dated 2013, I can't find a newer dated equivalent) a broadcaster must provide subtitling if it attracts a viewing share of more than 0.05% of total viewing.
And here is a BBC R&D article about subtitles speed from 2015.
https://www.bbc.co.uk/rd/blog/2015-0...d-subtitles-be