The major issue with Heathrow is that the runways are running at ~98% capacity which means as soon as anything goes wrong, an aircraft slow to clear the active, minor tech delay, fog, everyone has to start stacking. No other comparable airport is running that hot. By adding an extra runway you're basically going to ~65% capacity, that improves the situation and is better for the environment as you don't have aircraft waiting in the air or on the ground burning fuel. Plus you spread the noise pollution out over a wider area reducing the impact at any one location. You can also obviously increase the number of aircraft using the airport but there are plenty of restrictions the government can apply that prevent the situation getting as bad as it is now.So they are going to build another runway which means the pint pot wil get filled quicker
Gatwick could, and probably should, get a second runway, but is harder to get to from the rest of the country. so wouldn't have as big an effect without a big spend on transport infrastructure.
Expanding Birmingham etc. would make sense if the demand was there, but it isn't and there's not a lot of point building capacity where no one will use it. Like it or not Heathrow is the most popular airport in the UK for a variety of reasons.



