Ballpark figure for a new bus meeting TfL specs is around £275k (I think hybrid is mandatory for anything new, and for the central area now.) Boris buses were around £350k a piece, and are unsellable outside London (which is why they are all TfL owned, whereas everything else is operator owned or leased.) So for vehicles only a few years old, and the EU IV and EU V borises, it's the right choice. LGV and coach operators coming within the M25 will also be hit by the tightened LEZ mandating EU VI, so they may also be in the market for upgrades on newer or specialist vehicles (mobile cranes and the like.)
The LEZ and ULEZ zones are strictly for road vehicles, no legislation for anything else envisioned. So trains, canal boats, stationary generators and the like are all safe, at least for the foreseeable future, although new purchases of all these have to meet current regs. In terms of NOx emissions, the household gas boiler would be the next target after road vehicles.