While the M25 is extremely important to LHR a huge number of passengers come and go from central London and do not use the M25 at all and most of them use the two rail links available from London to the airport.
I am not saying further expansion will not put more rpessure on the roads but the issue id London needs a hub airport to continue to compete with Paris FRA AMS of those only Paris has two airports but there is little need to ever transit between them . LHR on the other hand has a huge amount to transit pax and needs them to support route development . And this si the problem , expanding LGW or STN does nothing at all to meet that need. Expanding Gatwick just allows more Easyjet and bucket and spade travel and does nothing for business. Stansted is Ryanair world , enough said.
So the Govt -oddly enough the Tories again- putting party before country -faff around forever about expanding LHR because of fear of loss of a few west London seats mostly in safe areas. pitiful.
as for cost overuns it never ceases to amaze me how this happens, for most of my career I ahve been involved wioth projects costing 500M to 1Bn USD so not gigantic but not small . Most have cost overruns but of the order of 5-15% , the kind of ones we have on Uk projects of x times the estimate are unthinkable and it is astonishing to me and many of my colleagues that any infrastructure project can suddenly end up costing three, four or five times the estimate and then the same contracting and implementation team goes on to get another major contract. Can't all be backhanders and freemasons surely