No, not the ordinary preschools, but the really nice ones, probably. It's hard to tell, because this was back in Portland, which, being a smaller town, had fewer preschools in general and only one or two really nice ones. London probably has tons. I just wanted to emphasize that there usually isn't a downside to getting on as many waitlists as you can - you can only benefit.
As a comparison, now that I'm living in Brighton I haven't had a single problem getting my daughter into a good primary school. Whereas again, in Portland, the better schools are all filled up a year before they would even start... sigh.
Good luck! I love school hunting.
Oh, and I'm sure it wouldn't be a problem getting your twins into the same class, especially if you put them both on the waitlist at the same time. You can make it clear that they're a double deal... actually, now I'm rereading your initial question, and it sounds like you're wondering if there's a policy that keeps them from putting twins in the same class... hmm. I don't know about that, but it seems unlikely.