Conformance is simply the fact that BA require you (at T5) to have completed all your "bits" and be through security at least 35 minutes before your flight departure time - if you arrive at security with less time than this, you are not allowed to proceed through security. The below is taken from the BA website:
Going through security
Please allow yourself plenty of time to go through airport security checks.
Please note - if you are travelling from London Heathrow Terminal 5 you must pass through check-in and security at least 35 minutes before your flight departs.
Increased security at most airports means that the screening of hand baggage and body checks can take longer, particularly at busy times.
Boarding the aircraft
- Please have your boarding pass and passport (plus any other travel documents like visas etc.) readily available when boarding at the gate.
- You will need to show your boarding pass to the cabin crew when you board the aircraft.
- If you arrive at the boarding gate after the gate has closed you will not be able to board the flight.
Please note - if you are travelling from London Heathrow Terminal 5 you must be at the boarding gate at least 20 minutes before your flight departs.