It's not a criminal offence to fail to do an age verification check.
It is a criminal offence to sell alcohol to children (people under 18 years old). It is a defence that either the seller took all reasonable steps to establish the buyer's age or nobody could reasonably have suspected from the buyer's appearance that they were under 18. (sale of alcohol to children s146 Licensing Act 2003)
It is mandatory (i.e. a requirement of licensing) for all licensed premises and those with a club premises certificate to have an age verification policy (The Licensing Act 2003 (Mandatory Licensing Conditions) (Amendment) Order 2014).
At minimum this requires 'the responsible person' (the person selling alcohol) to request identification (with photograph, date of birth, and a holographic mark or an ultraviolet feature) from individuals who appear to be younger than 18 years old. The Home Office has produced guidance including a 'model policy'.