When I raised this issue myself here in Flat Chat some time back Jimmy drafted a potential motion about staying dry until all the formal business of the meeting had been concluded.
I remember that. The problem is that the OP’s meetings take place in a licensed premises – you can’t stop people drinking outside the meeting.
Perhaps the solution would be to impose a “lock-out” so that anyone who leaves the meeting after it’s started can’t get back in. But that wouldn’t take into account late-comers and weak bladders.
A polite request backed up by the chair’s iron fist might do the trick. Standing orders, enshrined in a by-law, allowing inebriated members to be “named” in minutes might deter the tipplers too.
But I don’t think the idea that decisions made under the influence of alcohol are legally suspect is real – otherwise half the wedding engagements, business lunch agreements and Acts of parliament would be in doubt.