Choices are M4/M25 or else some back route through Bracknell, A322 to Guildford way and then A246 round to Leatherhead I guess. The motorway is likely to be quicker, even in rush hour I should think as once on the M25 you're going in the direction of less traffic generally as you move away from Heathrow. No idea on the time, but I used to do the M25 in the same direction from the A3 to the Redhill turnoff and that stretch of motorway could be done in 20 minutes at peak times.