Unfortunately, Kent itself isn't served by the tube, but I've managed to park at both Bexleyheath and Welling stations, and purchased a travelcard from there. The line goes straight into Central London (Victoria) and trains are fairly regular. Alternatively, You could try driving up to Essex and catching the tube from one of the stations there (Upminster, for instance). Worth mentioning as well is the National Express coach service which runs directly from Canterbury Bus Station, to Victoria Coach Station. It costs about a tenner return, and takes about two hours. Alternate coaches stop at Hempstead Valley and Bluewater shopping centres, so you can board there if you prefer.