There's no rules on how an air rifle should be stored...
But the Crime & Security Act 2010 (S.46) made it an offence for a person in possession of an air weapon to fail to take reasonable precautions to prevent any person under the age of eighteen from having the weapon. This legislation essentially relates to the storage of air guns and the requirement of owners to prevent unauthorised access by children. Failure to do so renders owners liable for a fine of up to £1,000. It doesn't say what the reasonable precautions are but reasonable should give you some guidance.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep