As Brekkie says, it's very likely to be the channel not the platform. If the programme didn't have subtitles on Freeview than it would very strange if there were subtitles on another platform, be it another free platform like Freesat or a pay platform like Sky or Virgin Media.
Ofcom's Code on Television Access Services
sets targets for the amount of TV subtitling, signing and audio-description that eligible broadcasters are required to provide. According to this document, the Drama channel was one of the channels in 2017 that was required to provide Level One television access services, that is full requirements: https://www.ofcom.org.uk/__data/assets/pdf_file/0016/81322/channels_required_to_provide_television_access_services_in_2017.pdf
Ofcom's advice on complaining about lack of subtitles or other access services is as follows:
Please contact the broadcaster directly. To improve the quality of access services it is helpful for broadcasters to receive feedback and complaints directly from consumers.
If you've complained to the BBC and are not satisfied with the final decision you’ve received, please complete our BBC complaint form
. If you’ve complained to another TV broadcaster and you're not satisfied with the response you received, please complete our TV and radio complaint form
Taken from: https://www.ofcom.org.uk/tv-radio-and-on-demand/how-to-report-a-complaint/subtitling-signing-or-audio-description
UKTV, the network of channels that runs the Drama channel, has a contact form you can use here: https://corporate.uktv.co.uk/about/contact-us/
(FYI I have used the UKTV contact form before and received a prompt response to my enquiry).