If you look under 25 they're legally required to request ID. ID has to be an official legal form of photo identification such as a passport or driver's license.

Some shops will enforce this stronger than others, especially chains like supermarkets.

Regarding asking strangers for cigarettes, when I smoked it wasn't uncommon for a stranger to ask me for a cigarette. As they've become more expensive it's felt like it's more of a taboo/rude thing to do. But I still get the occasional person ask when I smoke, but I kinda see it as a bit trashy to do so.

If you mean is it common to wait outside a shop and all people to buy you cigarettes (presumably because you're underage), don't do that. Best case scenario everyone tells you no, worst case scenario somebody takes your money and doesn't buy you the cigarettes. Nobody's going to casually break the law for a stranger.

If you're not underage and you're worried about your ability to buy cigarettes whilst visiting the UK, don't be worried, just keep your passport with you and you'll be fine, you may or may not be ID'd depending on how young you are/look but as long as passport says you're 18 you're fine.