When I took my business trip to Guildford, England in the beginning of 2006, I had the luxury of a spare day (it was cheaper to come home on Sunday rather than Saturday).
Although it was a fairly rushed trip, I spent the entire day exploring London and from what I told, couldn't have gotten any luckier with the weather.
I was actually in the city the very same day that many people may remember the large whale being found and trapped in the Thames River. Although I didn't actually see the whale, there was definitely a lot of commotion about it around town.