Are there any Brit TV shows of which the length is just about 20 to 30 minutes?

Well most shows like Eastenders, Coronation Street etc. will have some sort of accent which can make it a bit more difficult to understand so you could try Holby City/Casualty or you could try watching some Doentaries as well there's loads of different ones and they're insightful not just boring, mind-numbing telly. Put subtitles on as well it makes it a lot easier.