I stand to be corrected but I think PAXboy is talking about an exit route under the terminal but I can't see what that would achieve and if it had been put in where would it exit apart from onto the original approach road that went through the back streets of Luton. Also the CTA would still be the same size and that is the core of the problem.
The final boundary of the CTA was set when the cargo terminal was built and a taxiway was put in to serve it in I think the late 70's/ early 80's. The taxiway avoided the unstable council tip which I think in those days might have been outside the airfield. Today that land is the long term car park. Back then no one would have looked 30 years into the future but if that taxiway had been put in further out so it crossed the tip it would have transformed the size of the CTA but I can see why that would not have been considered.
I would hope that someone is now looking at capacity beyond 18 million as any work would be at least 10 years away and no doubt be south of the runway.