As mentioned above, Clapham junction is a good one. They run through the lower numbered platforms there mostly going via Richmond or Hounslow. Most of them run that side however I believe there are usually 2 or 3 diagrams via Wimbledon that are 707s. You're much better off sticking to the side via Putney anyway to spot one. Queenstown road may be a good island platform to scout them out however you won't be waiting long for one to show up.