We go to Leatherhead quite frequently, and have found there really is no one best way... getting to the A3 through Brixton, Clapham, Wandsworth can sometimes take ages, but equally we found that heading down the Gatwick route via Ikea/Purley to the M25 can be hit and miss, particularly because of the single carriageway beyond the big roundabout bit... on balance, we tend to go via the A3... Haven't tried the A2 route, so may give that a go next time... good luck!