What’s with the weird British pronunciations of words like Gloucester?

LOL silly you... you know we invented these words.. it ain't our fault if ferriners can't speak what should be a common language.  :P  And it ain't our fault if you guys can't be bothered to spell correctly :P

Same as any other language. In Russian, zdrazdvuitche is pronounced "strastitche". In American we have "gh" which pronounced a lot of ways but never "gh". 

Now here is a young lady who is going to tell you about her bum:

The shortened pronunciation may have evolved just out of laziness, or perhaps in some cases as a deliberate affectation by local people to distinguish themselves from outsiders.

There is a town near to where I live whose name is spelled "Slaithwaite". It is generally pronounced "Slathwaite" (never "Slay-thwaite") in the surrounding district; there is an alternative pronunciation, "Slawit", which rarely used except by inhabitants of the town or those with a close connection to it. This post was edited by Reverend Muhammadovsky at January 19, 2018 4:03 PM MST

So the American is mouthing off. "You Brits are goofy. You call the front of a car "windscreen" when we call it "windshield". That's goofy. We invented the car, you know."

The Brit replies, "That's true, but we invented the language."