So on the one hand you argue that Heathrow simply must expand to "improve connectivity" (whatever that means) and then on the other hand you argue that Gatwick and Stansted- neither of which are at capacity either- are perfectly attractive airports in their own right.
Of course Gatwick, Stansted, and Luton are attractive airports.
But they serve quite different markets.
If, hypothetically, a Myanmar carrier wanted to serve London, they would have one preferred airport. But it is full. The only way to get in is to buy/ trade slots. At the times they would want (Asian arrival into London means early morning is the best time), the slots are in most demand and most expensive.
If a Albanian LCC wanted to enter the London market, they'd rather somewhere like Stansted more set up for low cost operations.
Luton, Stansted and Gatwick are all constrained by infrastructure/ slots too.
Gatwick is the businest single runway airport in the world.
Stansted, for example, might have a fair amount of capacity on certain parts of the day, but no for a departure between 5.45m and 7.30am when any based aircraft would need to depart to operate profitably.
That's why they all want to expand and have private investors willing to pay for it.
Foulness would be nearly entirely on government land and close to the sea.
Awful location that just doesn't make sense given the 100,000s of jobs and thousands and thousands of businesses in West London and the M4 corridor that Heathrow supports.