Do you by any chance know that old British TV series "The Professionals"? I understand English pretty well and I even lived in UK for 2.5 years but for some reason this TV show is really difficult to understand for me - half of the time I cannot figure out what they are saying and I do not know why. Is it some local dialect or something?

There are more accents within the UK that have less in common with each other than most other English language speaking countries combined. At least it feels that way when you work in a few pubs there...

A lot of the regions of the UK have pretty strong dialects of English, not just accents e.g. the Doric Scots of my birth uses "fa", "fit", "faur" and "fan" for "who", "what", "where" and "when"

e.g. "far div ye bide" -> "where do you live"

However, I haven't spoken like that much since I was a wee loonie.

For instance, the accent of one of the main characters in Monty Python's Flying Circus. Which one is it?

Or any other std TV shows, which have more or less standardized accents, no?

I think GP meant the shakespearian accent, which is probably the same as the one used in american TV series or movies to sound british.