I'm also bemused when I hear in the UK something like "I'm gavering on Fursdays".
Like, you know it's spelled "gathering" and "Thursdays", right?
Note: I'm neither a Brit nor a native English speaker. I just have a casual interest in languages and how languages change.
Someone already commented that this is a case of th-fronting, but I would like to add that it's spreading relative fast all over England, Scotland and New Zealand in the (not so) young age group.
In fact, the th-sounds, which are dental fricatives, are quite rare among the languages of the world and not easy to pronounce. Only European Spanish, Icelandic, Albanian, Welsh and Greek have them. German had these sound, but lost them during the High German Consonant Shift, which resulted in a /d/.
Some interesting discussions/facts about this (totally non-scientific, of course):
https://www.reddit.com/r/linguistic...ome_ongoing_sound_changes_in_english/e0tgk4y/
https://www.babbel.com/en/magazine/th-fronting
Regarding your last sentence:
1) orthography always reflects the pronunciation of the past (take the now silent k in knight as example)
2) English is a polycentric language, ie. not one standard exist from which errors can be made
3) people speak as they please, writing is just a representation. Otherwise, languages wouldn't have evolved and would not evolve in the future.