On £50 notes: it's not just that some places won't accept them, it's that you're very unlikely to even see one. I've seen perhaps two or three £50 notes in my life, and never touched one. Banks just don't give them out; if you ask for several thousand pounds in cash you'll get twenties. I'm pretty sure that gangsters and foreign tourists are the only people who ever come to possess them.
500 euro notes are almost illegal in the UK, as there is no conceivable lawful use for them. (They're not illegal, but bureaux de change aren't allowed to buy them from wholesalers.)