I did say "a while ago", yes there was a Gatwick flight to Atlanta and by listening to the crew talking, they were way out hours having diverted to Stansted and returned to their Gatwick by road and believe me "Hotel mode" I didn't make it up.
But you may have misunderstood the conversation. IMHO cabin crew in BA are very aware of industrial agreements but not necessarily as aware of scheme (legal) limits.
They probably exceeded their industrial agreement but were within scheme rules. Had they exceeded scheme they wouldn't have been onboard. A standby crew would have been used. If no standby crew was available the flight would have been delayed until either a fresh crew was available or the original crew had achieved the required rest period.