A little more serious than duty on alcohol. In any event, all of the alcohol in Heathrow lounges is duty paid. Since it is consumed within the United Kingdom, it does not qualify for any exemption from excise taxes.

Comingling of passengers arriving and departing provides for the ability to:
1) undertake document swaps for improperly documented passengers to continue on to third countries; 2) use LHR for the purpose of transiting contraband; 3) pass items not cleared by security at LHR to pax who have been cleared.

The corridors and gate areas are all monitored by CC camera. Generally security staff will closely observe all pax arriving from suspect flights (i.e. points of origin with lax security) until they are in the connections centre or the arrivals hall.

However, lounges provide an environment in which it is very difficult to consistently monitor one or two suspect individuals. Also, the cloakrooms provide an ideal environment in which to perpetrate a document/contraband switch.

This is the non-arcane justification for prohibiting co-mingling.