The A322 through Brackers can be a crawl though, and to get to the M3, you still have to suffer one of the worst bits of the M25 (not quite the absolute worst, that'll be around Heathrow).
On the other hand, if you are within 30 mins drive of a Southeastern High Speed-serviced station (which over 50 percent of Kent's population is), you have a high speed service to London, plus 20 mins on the tube to either Waterloo or Paddington and a train out of there, it's an attractive proposition.
The only option where the car will beat the train decisively is places like Sevenoaks, where it's only around 45 to the M3 junction (when the M25 is behaving).