BA:
On short haul (LHR crew) you must achieve a 'minimum time sat at the table' at either Compass Centre or CAT (Central Area Terminal) lounge at LHR. I think it is either 30 or 40 mins that you must physically be sat at the table for you to achieve your break . If you do not achieve your minimum break time you will be removed from the next sector of your trip, or if they don't have enough standby crew to replace you, your next sector will be delayed to accommodate the meal break.
On long-haul our breaks are shared out evenly. On our LH aircraft (747/777) there are two sets of breaks '1st break' and '2nd break' so half the crew are always on duty. The 767's don't have the same rest facilities as the 747/777 so breaks need to be shared 3 ways instead of 2.