Yes, it is printed on the boarding card but,as I have been able to avoid T5 for the past 12 months the significance of the time in a box at the bottom of the sheet wasn't particularly clear. Departure and gate closure are two absolute deadlines that I would expect to meet. and 30 minutes was more than enought time to do that on Saturday.
Why would there be an issue with a security cleared (clean pax) mingling with arriving passengers, if they both have to exit in the same place? Usually the exit routes involve a series of one way 'gates' after passing each you can't backtrack. Domestic and international have difference regimes at differnt airports but, as for staff, why would any pax need to be escorted?
At the scottish Airports and MAN, LGW and A number of european airports 'clean' pax can exit from departures then re-enter the system at security to be screened again. At LGW BAA track you with photo ID. You can go in and out after you get to the departure gates - as if you were an arriving passenger. Where international and domestic depature lounges are combined, all departing pax have the same status and may or may not mingle with inbound domestic pax- however, international arrivals pax are always segregated.
Actually I will spend more time with BMI as they seemed every bit as good as BA whose quality has been on the slide for some time. In fairness I haven't actually paid for a BA flight in 2 years and am still using up BA miles! So I suppose I shouldn't feel too hard done by!