Really none of my business, but having been given a TV glimpse of the existing two runways and the central terminals, I’d have to say that it should be possible to build another runway in the middle. Build new terminals to one side, demolish the existing ones, and create another runway where they are now. If one existing runway were moved a little sideways (Just a few metres actually) it should be possible to fit in a fourth runway in middle.
Anyone ever heard of a jet plane underpass? To get planes across the complex from runway to terminals and back. A little too noisy, but conquer the noise (Reverberation) problem off the concrete and it could be done at the ends of the runways. Both ends. Clever noise reduction structuring, and Boris is your uncle, and auntie Theresa May and him are on a winner?