letter sounds something like “le-uh”
Ah. You're all talking about the Cockney accent. This started as a geographical accent, but has become more widespread in different parts of London. An American using that type of speech would qualify for speech therapy, but theirs is a standard accent so it is acceptable.
They have 'f' for 'th', 'v' for 'TH', 'w' for 'l' ('towow' for towel), drop their final 'g's in 'ing' endings (readin'), drop their initial 'h's (orse), drop their final 'r's, AND drop their 't' and 'k's in the middle of words (Mana-an, wha-evuh for whatever, le-uh for letter).
They also use what we would call grammatical errors, BUT they have a really cool rhyme based slang.