Last time I was in the departure lounge at Stansted I found a passageway next to Boots that seemed to lead into the Domestic Baggage collection area. Would I be right in thinking that if you arrive on a Domestic flight you can get into the departure lounge without the need to go right through and then back through security control. After all, you will have been through UK security control at another airport ?
There's a desk at the end of the domestic arrivals lounge with a BAA body making sure people don't wander left into the lounge. I think he's also there to make sure people don't collect hold baggage and walk back.
He/she has no ability to scan the barcode on your printed off boarding card so I suspect they would not let you down into the lounge and send you back through security.