My MAN OLCI boarding card for the 0700 LHR flight a few weeks ago doesn't quote a 'clear security by' time (Unlike the onwards T5 one). It does specify the gate closing 10 minutes before the flight is due, so that's 0630.

If you don't have Fastlane access, the security queue at that time of the morning can be up to 10 mins.

But I would have to say, why take a risk ?

I would certainly aim to be in the terminal, no hold luggage, OLCI ticket in hand by 0600.