I just had a "connecting" flight done through two separately booked tickets (the service I use did the bookings for me automatically).
I was expecting the airport I was connecting through, Birmingham, to not have facilities to allow bypassing security, but to my surprise it had. For my landing though all the doors were locked and when I asked someone working there, they got told that it was not possible to let us through and I had to go trough security again.
Why would the airport force me to go through security again? I was coming from another UK airport, so I would expect security standards to be the same.
P.S. Some of the suggested questions do not fully apply here because I wasn't moving between two different security borders, I was coming on a national flight, which I would expect to make a difference.
EDIT: to address comments. The doors I'm talking about were doors indicated by the " connections" signal right after landing, before reaching the baggage claim area. The presence of the "connections" signals makes it pretty obvious that the airport has such facilities, and the person working there was unsure why it was closed, they had to ask via radio if they were allowed to open them for us, which they where not. To be more clear, it was two different PNR, but two easyJet flights. The other flight was from the same terminal. (I believe there is only one?)
EDIT2: fixed my example as the change was at Birmingham and not Manchester as I initially wrote.