It might be worth remembering what Ms Greening had to say on the issue recently:
So we are using the claim that Heathrow isn't safe to promote a runway layout with serious questions over its own safety. Not to mention that if you get into trouble but don't make the runway, you are straight into water.
I wouldn't usually want to bring up safety as an issue, as it is the last thing that should concern an incredibly safe industry, but I'm really not convinced terminals at ends of runways are going to work.
What about MAD? Would there not have been an 18C/36C left open if this proposition was a safe one? Where might Spanair 5022 have ended up if it had departed 18R?