It's a funny example but to be fair to the British that also happens in other Germanic languages and it also happens a hell of a lot more in Romance languages.
https://m.youtube.com/watch?v=uYNzqgU7na4
TLDW; We have multiple different root languages in English city names, and you are only thinking one dimensionally, with the Roman Latin root. Cirencester is a good example, pronounced how you might expect while further north the town names get more germanic, and eventually Scandinavian.
The video helps it makes sense.
I've got a fairly plain accent, fairly close to Received Pronunciation or BBC/Oxford English, Americans never have a hard time understanding my pronunciation, and even I sound closer to 'aluminyum' or maybe 'aluminiyum' more than a clean 'aluminium', though it still sounds much closer to correctly pronounced British word than to the American version.