OK - this is a simple one..........I think!
Passengers from an International airport are not security screened to UK DfT requirements and therefore the airport is legally required to keep them separate from all other passengers until they are either landside or re-screened for an onward flight.
If you go on a Domestic (or International now) flight that departs from the Satellite that you walk across the bridge to then there is no reason why you couldn't walk back down the tunnel and turn left back to the Departure Lounge - you may be challenged to confirm why, but you can still do it as long as you have your boarding card. The security guy there is to prevent arriving Domestic passengers from going into the lounge. I guess the BAA might actually like that as they could sell more, but I think it's primarily a customs requirement.