I've submitted a complaint to Transport for London. They've been quite receptive to some of my previous complaints about buses, e.g. the driver who was drinking from a can while the bus was moving (i.e. he only had one hand on the wheel and his head was tilted back so that his eyes weren't on the road), so hopefully they'll be able to have a word with the person responsible.
Meanwhile, I wonder what I could have done differently, i.e. how I should guard against a similar situation happening again, but I haven't come up with any useful ideas. The obvious approach would be to wait until the oncoming vehicle has committed to the turn (in case they're lying), but at that point I don't think I'd have time to get all the way across before the following traffic comes through (assuming that I'm stationary, i.e. have no momentum), so I'd still have the problem of playing chicken with oncoming vehicles. I could wait until there's a gap in the traffic, but then I might wait so long that the light turns red again. Also, I need to be aware of traffic behind me (also wanting to turn right) who might get impatient. For now, I think I just need to keep my wits about me, and be prepared to take evasive action when necessary.