Why Whitstable? To be honest Canterbury is pretty small, and you could probably do it on same day as Dover?
Stonehenge - very overrated. If you really must go, then I'd suggest looking at visiting Salisbury, or alternatively, go from Bath - Wells - Stonehenge while you drive back up to London. You can see Stonenhenge from the A303 and to be honest its as good a view as visiting it!