Only used it for US and Canada. Didn't know you could use it for the UK!
Can't answer your question on when is best to book. I went to a forum and found an "is this a good price for" thread on the same hotel/date period we were going. Someone said yes so I booked, I figured a good hotel for the price of just a bit more than a travelodge it was worth the risk!
edit: aaah, I see they do London!
To work out if it is any cheaper you need to know which hotel the offer is on and then look around yourself to see if you can get it cheaper!
The **may** be of use.
http://www.betterbidding.com/index.php?showforum=175&http://www.betterbidding.com/index.php?showtopic=12693&