I love Paris - but the few hours you'll have there won't even begin to scratch the surface and will be a full day away from London. Six hours getting to and from trains, then sitting on them, are way too much. Save it for the next trip.

The good news about watching for traffic is that most street crossings have "Look left" or "Look right" painted on the roadway as you step off the sidewalk (pavement to Brits). It's not there so much for tourists as it is for locals who may not know that a given street is one way, but it helps. (I was once pulled out of the way of an oncoming car in Russell Square by a blind colleague. I forgot which way to look, but he knew which way to listen.)

Only other thing I have to say is that the Churchill War Rooms are a must-see.