Me and Mrs I used Metropolis (under the arches at Vauxhall, right by the station. Google for website and rates, times, etc).
I suspect that we could have saved a couple of quid going elsewhere (less central), but for my money they were very good. I had (instructor) Chris, who I particularly thought was very good as he managed to combine his obvious years of experience on bikes that would scare the carp out of me with an attitude that scooters are perfectly viable modes of transport and not silly things to be laughed at (which might not sound like much, but not all traditional bikers behave like that).
We hired scoots from Metropolis to do the CBT, and they were in reasonable nick, and it was good to do your road-based bit of the CBT on roads we would soon be riding around (Kennington, the Embankment, etc).
If you do find anywhere nearer/cheaper, then please put up some comments as I'm hoping to do my full auto test later this summer (and unless someone tells me about anywhere significantly better I'll happily be off to Metropolis again.
If you want to ask anything about doing the CBT/getting your first bike, then there will be loads of people who know more than me, but if none of them will talk to you then I'm happy to tell you what little I know based on a year's commuting through town on a 125.
Good this forum isn't it? I had the opposite experience with Metropolis. The bloke that took us told us that scooters were dangerous and our actual on-road bit of the test was no more than once round the block (though this was a big block in rush hour). Still we all got through with this method except for the one person who was pretty much abandoned afger she hit the playground wall in the first half hour. I did my full test as the last customer of Greenwich council's now defunkt scheme and that was first class. I know that Southwark (I think it was Southwark anyway) used to operate a similar set up based down at Crystal Palace as my girlfriend did her CBT there. It was a course of saturday morning classes. That might still be running.
I had a very good experience at Metropolis in Vauxhall and thought they were pretty good tutors, willing to put in the time necessary to get you feeling reasonably confident ( I was learning on a motorbike) about controlling your machine and surviving the London streets...I've also heard very good things about Elite (but they're wandsworth way) and also Flying Colours in Dulwich. Good luck with it...