Best deal is on Southern from Victoria. Trains take slightly longer than the Gatfraud Express, but IMO they are more comfortable and most importantly a single to LGW can be as cheap as £3.00 if booked in advance on http://www.southernrailway.com/

If you live in W or SW London, within easy reach of Kensington Olympia, West Brompton, or Clapham J. stations, Southern is a definite no-brainer for price AND convenience.