The guided tour is an integral part of the journey.
The Original London Sightseeing Tour costs �18 on the day or �16.50 online:
http://www.theoriginaltour.com/
The Big Bus Company charge �20 on the day or �18 online:
http://www.bigbustours.com
Personally, I don't recommend the open-top buses. You can get around London far more cheaply by buying a Travelcard for a day of unlimited travel on the buses and tubes. All of the attractions which the open-top buses pass are within the Underground Zones 1 & 2. A day ticket costs just �4.90 (or �6.20 if you want to travel before 0930, Monday to Friday):
http://www.tfl.gov.uk/tfl/fares-tickets/2006/o ne-day/travelcard-day.shtml
If you need any help planning a route, using the buses and tubes, around central London, just post again and I'll be happy to provide some suggestions.
Chris