Why not wire the main corridors, such as those used by the main routes from Central London, and have bi-mode, so that they could use the wires on the main sections of route but can use hdyrogen or hybrid or whatever, on diversions & extensions off the main routes?
Because, for the same reason the bi-mode IEP is a bad idea, you'd have a bus hauling dead weight in the form of its engines/batteries/fuel tanks around for a large amount of the route, along with increasing complexity and maintenance costs tremendously. Except, in the case of a bi-mode bus it'd be even worse, because you'd be starting up the engine and stopping it again within a few minutes (or even seconds). Lots of wastage there.