HMRC at Gatwick provided the following advice to our airline.

On non-EU departures, ONE SINGLE BONDED BAR may be opened for the purpose of serving pre-departure drinks to premium cabin passengers only and no longer than 1 hour prior to scheduled departure time. Once the door is closed, all bars may be opened but with the caveat that if the aircraft does not actually depart for a non-EU destination (eg. cancellation), the full value of duty and taxes MAY be charged upon all bonded items in the bars that are opened (including the single designated bar for the premium cabins).

On EU departures, there are no restrictions at all. Anything may be served at any time as the relevant duty and taxes have already been applied.

A flight that operates first to an EU point and subsequently to a non-EU point (eg. LGW-MAN-YYZ) may be considered as a non-EU flight for the sector within the EU provided that there is no local intra-EU traffic uplifted.

In practice, I've never known HMRC to levy duty unless there was legitimate evidence of abuse of the spirit of the regulations (eg. bars opened and bottles snuck out to ground staff, etc..). This regulation is also suspended informally during the week before Xmas, when numerous bottles of various stuff are sneaked off for the loaders' Xmas party.