Perhaps living until very recently in the Netherlands has spoiled me a little, but I can't help think most British airports (except really Manchester and perhaps Stansted) are a little constrained in their transport links. Schiphol, Frankfurt, Cologne, Brussels Zaventum, for example, are integrated into the transport network of the country, which trains passing through to a wide range of destinations. The result? I would never think of driving to an airport. I flew to China a few weeks back out Frankfurt. My plane ticket was also my train ticket (Emirates), and when I fly to Iceland in the summer also out of (forgotten airport for the minute) my plane ticket is my train ticket, which trains straight to the airport.
Manchester stands out, having Crewe, central Manchester, Liverpool, Leeds, York, Glasgow, and Edinburgh on its route map.
Stansted as well, has not just a train to London but also trains across to Cambridge, Leicester and Birmingham.
I've only ever transited through Gatwick, so I don't know all that much about it.
Heathrow is totally isolated! Just a horrifically expensive (most expensive?) airport line to Paddington or the very slow trundle on the tube.
So what does this tell us? Perhaps that Stansted is cognizant of its position as a low cost Ryanair base for much of the midlands (Birmingham airport is OK, East Midlands is useless) as well as London? The flight to Maastricht used to go from Stansted and ran for a few years. It was never overwhelming, but it kept going. Then it transferred to Flymaybe and went to Southend. It was dead within 3 months. Nobody could recognise Southend, and explaining to confused Dutchies at the airport that £78 return (for several people) to London was indeed justified as were still somewhere near the Netherlands...
And that Manchester really has an amazing geographical spread? Or is it just a useful place to turn back trains so they don't clog up central Manchester? Or perhaps because you can fly to America, Africa, Asia, and Near and Far East Asia without changing? The new MAN - HKG route is great!