This may sound like an obvious advice, of course all passengers should arrive at the airport early with sufficient time for check-in, security control and possibly passport control to make the flight and to avoid delays for other passengers. But at London Heathrow there is a much more important aspect of this to keep in mind and it is called ‘conformance’.
This means that the airport authorities at Heathrow airport have decided on a cut-off time when all passengers need to have left the departure hall and entered the security control area. Conformance will be checked when the boarding pass is scanned.
Currently the cut-off time is 35 minutes before the departure time of your flight. So if your flight departs at 7.00 am you need to scan your boarding pass at 6.25 am at the very latest. If you scan your boarding pass at 6.26 am there will be an error message, the automatic gate will not open and you will need to contact a member of staff. In this case they may deny you to proceed which means you will not be able to board your flight. You will be asked to rebook to another flight or buy a new ticket, depending on the ticket type and the circumstances. If you are a transfer passenger and you miss the cut-off time because your incoming flight was late it is up to the airline to rebook you, assuming both flights are on the same ticket.
In some cases the staff may decide that it is highly likely you will make your flight anyway. Perhaps the flight is slightly delayed, you have access to fast track security (which may speed up the process) or it may be the last flight of the day. Then the officer needs to make a phone call to get approval. A late incoming flight may also increase the chances to pass later than 35 minutes, but there is no guarantee as it is really up to the staff to assess the situation.
The conformance check is done when leaving the departure hall for the security control or when passing the flight connections centre just before the transfer security control. It is used in all terminals at Heathrow airport and in particular at terminal 5 where most British Airways flights depart and where there are a large number of transfer passengers.