A few seasons back I had occasional access to a Corporate Box at Spurs and they were very strict against taking drinks from the Bar/Dining Area to the seats at the Glass Windows - even to the point of having signs made up to that effect. Indeed, if you sneaked a bottle with you and were seen sipping from it, they asked you to withdraw further inside.
Last season I was in a Box at Twickenham for the Premiership Play-Off Final and we were allowed to drink within the Box itself (from where you could see the pitch handy enough, as it happens), but we were told
not to take our drinks out to the little open-air balcony in front of the Box.
As for elsewhere in the football stadia generally, there is a strict policy against taking either bottles or glasses (both only available in plastic) from the bar area into the seating or standing area. In fact, at Spurs they have Stewards at the steps, with big bins alongside to dump your glass (even if half full) before you go back out to the seating area.
It's the same at Griffin Park, and every other League Ground I can remember attending (nearly 70 of the 92) i.e. it is prohibited to drink alcohol either on the terraces or in the seats.
So unless the Rule for Corporate Boxes has been relaxed recently, then anyone within these who drinks alcohol within sight of the pitch is strictly speaking breaking the law. As for the enforceability, Corporate Boxes often have Waiter Service, so that they (or the Bar Staff) may simply refuse to serve more alcohol in the event of non-compliance.
Edit:
In fact I've just found the following:
http://www.flaweb.org.uk/docs/faqs/faq011.php
The Sporting Events (Control of Alcohol etc) Act 1985 prevents: -
* drunken entry into a football ground (which, in practice, to be an arrestable offence includes disorderly behaviour);
*
the consumption of alcohol within view of the playing area including, during the restricted period (15 minutes before the start of the event to 15 minutes after the end of the event), rooms within the ground from which the event may be directly viewed;
* the consumption of alcohol on certain coaches, trains and motor vehicles travelling to a designated football match;
* the possession of fireworks or flares.
The Act applies to the following sporting events:-
* Association football matches in which one or both of the participating teams represents a club which is for the time being a member (whether a full or associate member) of the Football League, the Football Association Premier League, the Football Conference National Division, the Scottish Football League or Welsh Premier League, or represents a country or territory.
* Association football matches in competition the Football association Cup (other than in the preliminary or qualifying round).
The Act only applies in England and Wales.