When buying age restricted items such as alcohol or entering
licensed premises, such as a bar, pub or club, you may be asked to
prove your age. The current scheme that shops and venues operate is
called Challenge 21 whereby if you look younger than 21 you may be
asked to show ID to prove that you are over the age of 18.
In some areas this is enforced further by the Challenge 25
scheme where you will be asked for ID if you look under 25 to prove
that you are over 18.
Here are some official acceptable forms of ID:
Driving Licence
A full or provisional driving licence is acceptable ID.

Passport
A passport is acceptable ID.

PASS Card
The Proof of Age Standards Scheme (PASS) is the national
proof-of-age accreditation scheme, endorsed by the Government,
which sets minimum standards for proof-of-age cards.
Around 1.5 million young people hold proof-of-age cards bearing
the PASS hologram and numbers are increasing all the time.
If you do not have a Passport or Driving licence and don’t
require one then the PASS cards are a cheaper & approved form
of ID.
http://www.pass-scheme.org.uk/
