A Scots fiver or tenner is usually OK in England. A £20 often tends to get suspicious looks. No-one ever uses £50 notes in the UK (or not for everyday purchases anyway) - £50 notes used to be the favourites of the forgers, and even though they've now had all the new security stuff put on them, most businesses simply won't accept them now.
Bottom line is that businesses are entitled to refuse to serve you if they want - they don't have to take your money. And all a banknote gives you is the promise to exchange it for a given value of coins at a bank, and you may not trust that promise if you don't recognise the organisation making the promise.