Yeah, no, I beg to differ, unless you’re someone who’s watched too many American movies, like just go to any spoons and listen to how the avg Brit actually talks.
There’s a very discernible difference between conversational British English and conversational American English, not just word choice, but speech patterns (this is academically evidenced btw). I used word choice examples because they’re easier to explain (but if you’re so pressed about it, you can easily look up studies on frequencies and difference of usage (as in, meaning-wise) of certain words in each country)
understandable but im brit too and genuinely considering their age group, their kind of generation does not speak like that often much older generations still speak like this but theres new lingo and slang now
plus i dont really get why it needs to be fully british english, i didnt even notice because who gafs

If you’re gonna set stuff in the UK, at least have the supporting cast use British English. Small localisation effort, big payoff (e.g. Bad Apple).
Not ‘school break,’ it’s ’school holidays’ or ‘half-term,’ not ‘about damn time,’ but ‘about bloody time,’ not ‘she saw him on the TV,’ but ‘she saw him on the telly.’