If you are viewing via the UK’s ITVX service, simply click the "S" or "CC" icon in the player settings.
Often the fastest site for TV-specific subtitle releases. 🛠️ How to Use .SRT Files McDonald & Dodds (2020) English subtitles
Look for SDH (Subtitles for the Deaf and Hard of Hearing) if you want descriptions of sound effects, or standard subtitles if you only want the spoken dialogue. Which Season or Episode What device or app are you using to watch the show? If you are viewing via the UK’s ITVX