There's always buses. They take ages, but they leave from the 'Central Bus area' near Terminal 1. Night Bus Number 9 comes to mind as a particularly useful one that goes to Central London. Note the night route is different from the day route. I believe Coaches also leave from an adjacent area to the Central Bus area. Coaches would stop few or zero times, some would go straight to Victoria Coach Station next to Victoria Station in London as well as to other destinations inside and outside of London.