Thanks Barney. As you say, nobody could accuse me of not having done some research. My argument at this point is I've done all I can to figure it out, now I'll try it once and see what I think. I've flown the M25 route many times, so I'm not worried about being declined the transit on any particular trip. As AlanM mentioned though, SVFR doesn't absolve you of the need to land clear.
As for rustle's 100' - he is certainly more experienced than I am and likely much more skilled, but there are other factors that make me think this is reasonable. For a start I cruise at 130 and glide at 70, so I'd gain quite a few feet during slowing down - and if my engine has failed I'd say to hell with the LTMA and climb from 2400 to whatever I could reach. Also, my understanding is that engine failures are rarely instantaenous if you're managing things properly, so this too would give you some time to prepare (and quite possibly fly clear of the questionable areas).