Until the last couple of decades, most British movies/TV tended to use a lot of “Received Pronunciation” English (the Queen’s English) which has very crisp enunciation. Other accents were usually relegated to very specific character roles that highlighted the rustic “cockney” or “Yorkshire” character.
More recently, other British accents have been used more and many of them are the opposite of crisp enunciation. Some seem to be talking with marbles in their mouths. This seems to represent a greater democratization of the characters represented with less of an emphasis on upper classes.