A few facts:
Anyone that has flown over London on a clear night can identify LHR: Two parallel runways , east/west with a dark parallel area to the north of it.
The real-estate has already been purchased , I suppose.
By the time the first aircraft takes off on the new rwy, all the big noise makers are gone.
( Ca 20 years ago we transited from Norway to Winnipeg and had to have a night at an airport hotel. In the morning we were up and getting ready on a nice and sunny day, then this THUNDER started. Me and my 4 year old ran to the window and got a great view of the BA Concorde taking off. All worth the expensive hotel.Now that was NOISE!!!)
I also do not understand why the parallel offset rwy proposed in STN is not build with a 300kmh speed train.
EVERY new airport build in the world has a Downtown highspeed rail connection.
We opened ours in October 1998 at GRM Gardermoen , now called OSL after great protracted controversy , sabotage of metdata for alternate site Hurum and a murder!!!
But all agreed it had to be done. So we did. Simples.
Anyone that has been to OSL knows that this is well functioning airport, with less then 23 min train the 55km to Downtown Oslo, every 15 minutes.
Anyway, I feel sorry for You Brits, and please take this as constructive criticism, but, You seem to be experts on other lands and peoples problems , and totally unable to see what to do in Your own back yard. Try at least to approach modern standards.
Its all well and good in small villages to stay in the past and attract the odd tourist , but The Empire that brought transport to many a corner of the Earth is now stacking inbound traffic into the stratosphere inbound Heathrow.
And one last on, that I could regret," After a Hard Brexit there will be no need".
Anyway
Good luck