Of course Oxbridge is hard work, it's not like applicants don't know that already. ^o)
It just means that student life is different, as you said, but not in a negative way, as you insinuated. Personally, I'd get bored if I had one lecture a week and two essays a term. I'd certainly not leave university with any sort of decent education, save what I taught myself. Student life at Oxbridge is intense, to say the least. We work hard, yes, but we also play hard. Oxford has an amazing nightlife for the size of the city, and most students go out at least once a week, some much, much more, some less. The work load does not stop anyone from playing sports! Just look at the league tables, and you'll see that we're doing well in almost every sport. I hardly know anyone who doesn't find to go for a run, a cycle, or to the gym or pool, either.
You've got to keep an eye on the long-term as well - a degree from Oxbridge stands up far better than one from a less academic university, and with good reason. We are worked far harder here than anywhere else, and it shows in the essays that we produce and the quality of the graduates that leave here and Cambridge. You say that at LSE they were only talking about work - that's only reflective of the students that were showing you around. If you couldn't cope with a degree that requires a lot of work, and self-motivation, then of course Oxbridge isn't for you.
In short, yes Oxbridge requires a lot of commitment, but if anyone who applies doesn't know that already it would surprise me immensely, to say the least.