I've been told that a pound is a good number for rides under 10 pounds, 50p or so for short 4-7 pound fares. As far as the distance from the Park Lane to Traf Square, I've taken that walk half a dozen times in the last 6 months, and it's never taken me more than 20 minutes even with a little window shopping at Forntnum and Mason along the way. Except for the time I stopped to...no, that's another story, and not for this forum