To be honest, I don't have personal experience with many writing course providers in the UK. However, I believe I may have come across an online learning platform called ELVTR that offers a program related to what you're interested in. If I recall correctly, they have a course specifically focused on TV series writing. I still have the link saved: http://uk.elvtr.com/course/tv-series-writing. You'd likely need to check out their website yourself to get all the details on the course curriculum, format, and any requirements. I haven't taken any classes through ELVTR, so I can't speak to the quality of their instruction. But given your interest in developing your TV scriptwriting abilities, it could be worth exploring their offering. If you do end up looking into it, feel free to share your impressions with the forum.