There is an awful lot of knowledge snobbery on this subject ( You don't know some random placename pronunciation that I do, you are inferior...despite never having previously known of the place's existence, let alone needed to know how to pronounce it*!), including on this thread where there are examples of people laughing at a place being mispronounced without actually saying how it should be pronounced (because every clever person knows right!). Which is especially unfair when the place is either spelt or pronounced "wrong" - eg Wymondham
Also a lot of the "wrong" pronunciations are arguably a matter of accents and "lazy" (for want of a more technical word) speaking - ie is Slaithwaite being pronounced wrongly or just more "properly", same for Hackney v Acne. I guess the question would be 'is the "correct" way really the way the locals say it, or are they using the equivalent of slang'.
That leads on to whether it is correct for an outsider to pronounce it more 'properly' or in the colloquial manner of the locals. Is a Londoner pronouncing northern names in a local accent being better, or are they doing the equivalent of speaking in English and using Pa-ree, Muenchen, or Kobenhavn? I wince a bit when a southerner pronounces Newcastle or Glasgow like a local - it doesn't sound right, almost like they are trying too hard. Same as a northerner saying Bath like a southerner sounds a bit like they are taking the mick.
*also, like a lot of technical language, place names can be one of those awkward things where you have read them a thousand times, you know what they mean/where they are, yet you have never heard them spoken and are unsure whether the pronunciation you have in your head is correct or just something you invented when you first read the word/placename. This is even more of an issue now so much communication and information comes via screens.