To get an idea of car journey times I suggest you take a look at Transport Direct, which is a powerful (albeit slightly clunky) journey/route planner provided by the Department for Transport. It provides info on both public transport and car journeys

One crucial difference is that it "takes expected traffic levels into account so that you can plan car journeys more realistically" (see see FAQ 2.11) - the other online route planners don't seem to do this (some satnav systems do of course).

Lewisham to Stonehenge is a bit awkward - Transport Direct's advised route does differ depending on the time and day you're travelling (either through London and then out to the west, or east-southeast out to the M25 orbital motorway and then clockwise all the way round to the M3 - the latter's simpler though quite a bit further, and initially takes you in the wrong direction!), however the planner's default route seems to be 'through London' one (which is what I'd do, but then I live here!).