You'd be surprised how quiet some popular parks can be if you go on the right day. A cloudy, insignificant day with poor weather forecast or it raining during the night or in the early morning will seriously decrease attendance at the park and will have very little negative impact (if any) on your day as long as you don't mind a bit of rain. Recently, I've found this particularly true of Thorpe Park and Chessington.
I've also found some medium-level rain at 3/4pm or even a short storm can be really beneficial - many families will often turn in for the day at this point, leaving a potentially dead couple of hours of ride time once the weather clears. I found this at Thorpe Park a couple of weeks back ... Colossus was a walk-on, even for front row!
Burniel | Great at theme park knowledge, not so great at designing signatures.