I hate LHR because it is so big but also love it because it is so big.
Crackpot idea of a new airport in middle of Thames estuary is just that.
LHR employs directly or indirectly well in excess of 100,000 people. Suggesting a new airport miles away means you have to provide accommodation / transport and services for another 1/2 million people close to the airport because with families and dependants that is minimum that will be required. Most people won't move but you will still need people to do their jobs.
People talk of the environmental impact but shoving a million tons of airport in the middle of the estuary will basically ensure that all the coast on both sides starts to flood at each and every tide with a probably impact further back along the Thames in London as its flow will be disrupted and it will back up.