I thought it was just an easier ID number for the vehicle (rather than the number plate) to make it easier for driver swaps mid route (e.g. "At 11.30am wait at Stop X and swap with bus 36" given that buses get rerouted, delayed and otherwise messed around with all the time.)
One of the comments here: http://www.londoncyclist.co.uk/how-to-report-bad-drivers/ claims it's a driver number, but I've seen drivers being swapped mid route and it not being changed.
I thought it was just an easier ID number for the vehicle (rather than the number plate) to make it easier for driver swaps mid route (e.g. "At 11.30am wait at Stop X and swap with bus 36" given that buses get rerouted, delayed and otherwise messed around with all the time.)