Drive - which as the journey is 143 miles to LHR and 163 miles to LGW each way - I think I can rule out cycling.
Just think of my carbon emissions when I use to read the FT in the CDG lounge on a Saturday morning.
Parking at either the Business Long Stay for Terminal 4 at LHR, or Parking Express for the others depending how long I am away.
Often park if overnight at one of the Holiday Inn's at LHR and catch hopper back to car when I return.
Short term at LGW before they closed it on the special weekend deal. However the Long Term at LGW North is very good and the buses fast and reliable.