So a friend of mine (also british) just got fired from their job for not "meeting their standards" (they've been there a year)

They've been there for a year, it's in their contract that the workplace needs to give a weeks' notice before firing someone... and in a week they turn 21

Obviously minimum wage goes up when you turn 21, so I think it's fairly obvious what the reason is for the firing here... even ignoring the ageism of younger people being paid less for the same work, I said to them "surely there must be some protection, like theyd need evidence of a performance drop or something, otherwise they could fire  anyone for any reason and say it was because theyre not meeting standards" but um... nope

My question is, can employers really just fire you for any legally protected reason (eg disability... a big fear of mine, as I am disabled) but just not say that's why on paper, and you have no protection?

I'm estranged from my parents too so I have no safety net... this is one of so many reasons i dont feel safe in work ngl, if i can't keep up and have the same level of productivity as my non-disabled colleagues (which, being disabled, I obviously can't) is there any kind of protection here in the UK? What would I be able to do, if anything?