In all airports of the world that I've visited to far, exit from departure area is close to security check, basically taking the inverse path. There, usually you will find also police officers to address if in doubt.
It is not necessary nor appropriate to ask airline to facilitate your exit. You have the right to leave at any time.
Then if the LGW airport is different in that, someone should explain exactly how and why.
Once, at Madrid I found myself in a situation where the corresponding entry/exit had been closed because it was very late in the night. I walked to an immigration check staffed by police, the officer didn't wanted me to pass in the "wrong direction" first. When I let him know that I had no alternative to leave, he let me through.