According to Ofcom, Amazon Prime Video has subtitles on 87.7% of its output. (There are no figures for Netflix as, being based in the Netherlands, it doesn't come under Ofcom's control). However that figure might be misleading in that it could, for example, be achieved by having 100% of TV shows subtitled but a far smaller number of movies carrying subtitles:
https://www.choose.co.uk/guide/subtitles-get-the-best-tv-service.html
Action on Hearing Loss (which is the new name for the Royal National Institute for Deaf People) has an ongoing campaign to increase the amount of subtitling on TV programming. Click on 'Timeline of the campaign so far':
https://rnid.org.uk/get-involved/campaign-with-us/subtitle-it/