Keep in mind that if there is a major attack, you may or not be able to get to the station for that train ride.

That's why getting on the Eurostar at Ashford might be your Plan B.

As usual, you all have given wonderful responses. It is great to get different points of view.

It looks like our choices are staying in central London and enjoying the sites as much as possible by foot, or changing our Eurostar tickets and renting a car to tour some of areas outside of London.

We are arriving at Gatwick and I need to contact Eurostar to see if leaving from Ashford is a possibility.

Would a car be more economical for a family of 6 than rail tickets?

Would love some specifics on an itinerary leaving from Gatwick.

Thanks again all for your help and I really do appreciate you not condemning me too much for even considering another option.

The first two times we were in London, we walked everywhere. It is quite interesting to walk. Just get a good walking map, we used a good tourist quide book, that has maps of each area.

PS We were all in our late 50's and early 60's.

Gatwick itself is out of London , it takes about an hour and a half to get to London by car (lots of traffic!!).
You can visit the South of England easily from there. There must be some experts round here :)

As for not being able to get to Eurostar train in times of incidents in London. On the 7/7 attacks Eurostar waved penalties for missing the train and allowed people to change times or dates by phone without penalty even on restricted tickets. Boarding at Ashford may be a good tip but if service is really disrupted there will be no trains leaving from Waterloo perhaps.

I was thinking more along the lines of an attack on public transport that might shut down the bus and Underground networks, which would make getting even across town to Waterloo problematical.

But if one were in Canterbury (for example) on the night before the France trip, one would have a better chance to get to London OR Ashford, depending on how the local situation played out.