Since 72D has clarified they are interested in just entering drinking premises as well as in ordering alcohol in them, it might be worth summarising the relevant UK drinking culture, and how it relates to age checks. I'm too old now for a lot of this to be recent personal experience, and some of the legal points are a bit different in Scotland and Northern Ireland, but I don't believe things have changed that massively.
Firstly, the legalities on children:
Under 16s must be accompanied by an adult when in places where the sale of alcohol is the main business (i.e. public houses, bars and nightclubs rather than restaurants). They can't buy or drink alcohol.
16 & 17 year olds are allowed unaccompanied. They can't buy alcohol, but can drink beer or wine with a meal if purchased by an adult with them.
Next the implementation. This depends a lot on the type of place and the time of day.
- Hotel bars will typically only ever check on ordering.
- Public houses (pubs) will generally check at the bar, except for places that get rowdy enough in the evenings (mostly in town centres, especially student towns) to employ doormen/bouncers.These days many English and Welsh pubs follow "Think 21" and card anyone ordering alcohol who looks under 21 to check that they are over 18. In Scotland this is "Think 25". A few places may be over 21s only.
Larger wine bars and cocktail bars are much more likely to check at the door, and to set their own higher age range.
Nightclubs all have door staff, and may have many, many house rules to be admitted. In particular large all male groups are known for being told to look elsewhere.
You have marginally more chance of convincing a doorman to let you in with non-standard ID (passport, PASS card, or EU driving licence) than you do of using it when getting served alcohol, since the in the first case the establishment isn't actually committing an offence and thus isn't at risk of losing its licence. However, if the place is busy enough to have a doorman, they're still very likely to tell you to go elsewhere. If you just want a typically British experience, without fuss or argument and without the risk of losing your passport, stick to smaller pubs, early on weekday evenings and to soft drinks. You may even get to experience a pub quiz or a meat raffle.