Hello! I'm a phonetician and accent specialist. I've written books about English pronunciation, it's regional variants and how accent is used to oppress those from lower socio-economic backgrounds. I've spent years researching and analysing accents of the UK.
It's so funny to me how the phrase 'The Queens English' is used to arbitrarily define a 'correct' way to talk, when lovely Liz herself evolved her speech system over the years - just listen to the Queen's speech in the 50's versus the noughties. Did you know speech was only standardised in a dictionary of Received Pronunciation in 1869? And then it was based on the speech patterns of the elite in the South, where we have our capital and centre of political power (at least at the time...!).
The reality is this 'PROPAH' speech exists only due to a series of invasions and marriages and reeks of the desperation of historical social climbing. No one cared about accent before people could escape their class.
Accents move and shift and they always, always will. There is no 'correct' when it comes to speech UNLESS meaning is lost. Swapping a TH for an F now and again will not likely make you misunderstood, whereas swapping an L for an X would be pretty tricky.
Please, PLEASE try and understand how accent is muscular. If I asked you right now to do the splits/a backflip - could you do it (anyone saying 'yes' I am v jealous!)? If the answer is 'no' that is the level of muscular shift you are asking a person to make to change a lifetime of swapping any speech sound for another. It's literal oral gymnastics. If I asked those of you who hate the F swap to do it for a whole day, how would you find it. It's really hard!!
Try and be tolerant. I realise this is a pointless plea to many, but try! These are just regional accents, they are not incorrect speech.
Incidentally, the next future evolution of Global English is merging into an international sound anyway. We now have a world where English is a common language for many speakers of different languages, so they communicate through English. This means "mispronunciations" are being cemented across speakers and countries. English speakers are starting to find themselves reinforcing these mispronunciations in the name of speed (think different stresses/over-pronouncing unstressed syllables) and these will become the norm.
Accents are a form of spoken oral history. We mustn't judge them (yes, even when the speaker is a teacher).