Like, what’s the point of its existence when that denomination is difficult to come by since ATMs don’t disperse them. I mean, will they still refuse a £50 note when paying for a £45 purchase? They go full counterfeit detection mode just because an individual has it despite that being the real one (not fake). Also, why does the Bank of England keep printing more of that when they know people barely use it anyway?

Because historically they were to high to be used much. So many retailers staff were less able to ID fakes. Hence fakes were created.
The. By the mid 2000s as prices got to the point hey were more useful. They were still high enough that card payments became common. So retailers rarely had the till cash to change them.