For ID to be valid it needs to have a good quality photo, your date of birth and a hologram.
Examples of valid ID are:
UK Driving License
Proof of Age scheme cards with a PASS logo, like a Citizen Card
You can find out about the pass scheme here.
Most premises run a check 21 policy. This means that if they think you look under 21 you’ll have to show ID to prove you're over 18 before they let you in. Some bars run a check 25 scheme too and it should say this at their front door.
But remember, whether you’re allowed into a venue or not is up to the manager and some places might only accept a passport or driving license as a valid form of ID.