I drive from just outside Guildford to Gatwick using the A25 through Dorking and Reigate. Journey takes between 40 minutes and an hour depending on traffic. There are plenty of back roads to avoid traffic. Over 5 years, I've never really had too many problems. Listen out to the traffic reports about congestion on the M25 between Leatherhead, Reigate, and Gatwick junctions, as people will come off the M25 and clog up the A25.