There is no T5 style conformance at MAN so you won't be stopped at security, as long as you've checked in online and have your BP in hand you'll be OK assuming the gate doesn't actually close.

I've had a few ridiculous close shaves, arriving at MAN 20 minutes before scheduled take off time and still made flight. Not recommended though.

If you have a particular reason for turning up late and need to be sure of quick security clearance you can book VIP Valet car parking which includes access to fast lane security.

Otherwise 6am would be sensible.