Thanks for the suggestions.

I just redid my calculations, and both options would cost about the same. Parking by Le Meridien would be 30 a night, and 10 a day for congestion charge.
Train for 4, parking and cab to hotel would roughly come out to the same.

Since I plan on doing lunch at my favorite spot in Benenden on the way over, would it make more sense to drive up to London instead of going back to Ashford?

I've been to London before, but never driven there. I do drive around Kent about 3 times a year.