I too have wondered this. I believe rcbos do meet the requirements of the regulations as they will switch the line and the main switch will be used as the isolator, but this does not answer your question.
The responder (GeoffBlackwell) in this thread (IET Forums - tt new db rcbo question IET forums) seems very knowledgeable and this is a response from him:
There are at least three points to consider here:
1) Most RCBOs are only single pole and will not, therefore, be suitable for isolation on a TT system, as this must be all pole.
Whether this matters or not depends on what you are calling the means of isolation for each circuit. Remember - isolation is required to allow work on conductors that would normally be live.
2) The second point concerns what method of shock protection is employed between the outgoing terminals of the main switch and the input terminals of the RCBO.
This is currently up for discussion at the Electrical Safety Council so my advice would be to ask the manufacturer of the DB.
3) If your TT system is derived from a TN-C-S system a single pole RCBO will not disconnect the supply neutral in the event of a neutral to earth fault (it will only switch off the line). This may result in your TT installation being connected to a TN-C-S neutral via the fault - not good if its an out door hot tub http://www.------.org/forums/forum/i/expressions/face-icon-small-happy.gif.
All in all I would fit an S type front end RCD but its your call.
Regards