It turns out that London just doesn't seem to be for me; part of this can be directly attributed to living in Tokyo immediately before, and knowing what I was missing. To wit: a (reasonably) cheap, efficient, and timely public transit system; rent about half the cost of London's; food worth eating; general hoofability around town; weather that consists of something other than "cold, grey, and dreary"; and a general sense of 'knowing how stuff worked'. That mental health support still seems to be back in the 1950s was a strong contributing factor as well. The overall culture and aesthetics just didn't gel with me, either. Nice place to visit if you're going museum or musical hunting, loaded with money, or you have lots of friends who're inculcated in London's jealously-guarded secrets, I suppose. But day-to-day life? Bleaaah. These are all contributing factors, however; it's really the job itself that drove me insane.
There's a very good reason that I don't work in the banking industry or do ueber-contracting gigs: I'm just not motivated enough by the lure of cash, and I care far more about enjoying what I do. My boss (an American!) was in the banking industry for twenty years, and had thus inculcated very strong notions of The Way To Do Things. These, alas, are directly opposed to the way in which I'm comfortable at doing things. I'm told he'd gotten significantly better by the time I was hired in, but it was still pretty awful for me. My coworkers, on the other hand, were (with very, very few exceptions) lovely to associate and work with; they're the main reason I'd even entertain the notion of working again for Google some day.
The environment in the London office seemed custom-built to be as bad as possible for me. The chairs didn't fit my stupidly-long torso (even after they ordered a custom seat back, it still didn't suffice). The main office section consisted of parallel rows of desks, with the monitors providing the only visual barriers; I could see every movement of the people to the sides and front of me, which was more than a little distracting. You could also hear pretty much everyone in the office, as well. (I think this indirectly led to our team not talking in person much.) The food in the cafeteria was free, which was nice, but still pretty highly mediocre. The vegetarian dishes weren't bad, though. I somehow managed to gain a whole bunch of weight.
The work itself was very different than I had expected and had somewhat been led to believe. No real systems programming, because that's just not how Google does stuff; things occur in the magic cloud, which is run by all google-specific software and languages and whatnot, all of which requiring their own hard-earned knowledge for which can take (literally) years to get approval into checking in to the source repositories. The job primarily consisted of babysitting Java code (note: I did not, nor do I now, know Java; this was made clear before I joined), or the google-bespoke infrastructure where the interface was mostly GUI and unintuitive. There was also a fair bit of python at work. I didn't know python before going in, and can't now say it's one of my favorite languages; to the contrary, I find it really, really annoying and designed by people who worship "elegance" at the expense of "consistent and straightforward design" or "getting stuff done".
A brief tangent: After talking with my sister about my vast troubles at work, it turns out that both she and I are somewhat dyslexic. This manifests in several different ways; we both have immense trouble not transposing numbers when writing them down, telling whether rows and columns are aligned or not is very difficult, and a tendency to misread words. Python is about the worst possible language for dyslexics, due to its use of whitespace as semantically-meaningful code-delimiting. In a more abstract fashion, google's cloud-control webpages were very, very difficult for me to mentally process; some things were clickable, some things were not; some bits led to subpages, some did not; some methods of interaction gave data that were usable in other bits of the access methods, some were completely inorthogonal and gave data that only worked in its little bit of the universe. And, naturally, it was all undocumented AFAICT. tl;dr: Google's infrastructure is very dyslexic-unfriendly.
My group's overall 'style' just did not fit with me. My purported mentor seemed to think I'd know all the proper questions to ask, and when to ask them. My boss later complained that I'd "Go off into the weeds" too often with my own exploration of the infrastructure. My group handled about 20 different applications, each of which required its own set of arcane knowledge and interactions with a gajillion bits of other google infrastructure, allowing me to not be able to hunker down and concentrate on any given thing. I thought there were immense amounts of stress in the air, and that people were really not terribly happy with their jobs; of the seven people who were there before me, one quit, two transferred, one is in the process of trying to transfer, and one is clearly unhappy with his job and manager. Only two people seemed to be happy and comfortable with what they were doing. This does not speak well of the espirit de corps.
And finally, my own personal situation: I was severely unhappy with what I was doing. My manager didn't seem to give a shit about me. I was so burnt out by work that I didn't have any energy to go out and do anything on weeknights or weekends. I became very, very depressed (to the point of suicidal ideation) with my own inabilities to pick up the job properly. To top things off, I had nowhere to go. Google's jobs are divvied up by location; you can't work on any random project, only on the things that exist within your local office. The only other local position I could have had was also managed by my group's manager, and so that was also right out. Google also typically requires a year's worth of work at a position before they'll let you transfer; occasionally, they'll let you move after only six months... but who in their right mind would accept an underperforming newbie who was told by his manager that I was dragging the spirits of the team down?
I kind of get the impression that Google is akin to the videogames industry in some ways, in which they'll take all the best-and-brightest eager young things they can get, work them mercilessly, and then sort of shrug when they get burnt out and quit. (Google also paid me a significant amount less than my prior jobs, not even taking the cost of living of London into account. Some people claim that the stock options and the benefits compensate for that; I, who've had stock options and ESPPs work to my favor precisely once, am skeptical.) Google also does not allow you to initially pick what group you'll work in or who you'll work with; the company line, AFAICT, is that after a year you've proven yourself and can choose for yourself.
All in all, the whole experience has reinforced the notion that with my extra-special-unique-snowflake attributes (depression, ADD, Asperger's, dyslexia, other stuff) and particular skill sets, I need to very carefully choose the sorts of jobs I like, will do well in, and fit in with. I don't believe Google is the sort of place where I could do that.