SOU has the railway and the motorway, but that also means it's quicker to get to other airports too
SOU to LHR (The world's busiest nightmare) might be OK by motorway but to LGW, by motorway, a nightmare! ... Quicker to go M27/A27, turn off by Arundel, thru Storrington to connect with the A24, then the A272 to connect with the A23 then the M23 etc. ..... And I knew these roads like the back of my hand and they ain't easy to navigate unless one knows where they are going.