It doesn't matter who sells the alcohol under your permission but even if they were to sign a disclaimer, it doesn't let you off the hook, if they were to sell to a minor for example.
Initially they would get into trouble for breaking the law but at the end of the day it comes down to you as you are the licencee.
You and your staff should always ask for ID - proof of age card (validate card/ citizen card) a passport or a driving licence with a photo & date of birth.
Nottinghamshire County Council - legislation I read states:
If found guilty of selling alcohol or allowing the sale of alcohol to under 18's (looking to be raised to age 21) and/or your staff could face a fine up to �5000 and contribute to you loosing your licence to sell alcohol.