As someone who does work on a startup in London, but didn't go last night, it's because I've found the HN London meetups to be chock-full of people who don't work on startups, but simply talk all evening about recycled opinions they've garnered from reading HN posts.
Not an interesting way to spend an evening.
I understand that you want to meet people that are more successful than you. You want to meet people that you can learn from. Or at least to meet people that are on the same level to you that you might share the experience with or work with. But that's exactly the same reason that people with less experience go.
So I'm not saying there's anything wrong with being hypocritical because this would be missing the point: you judge the evening on its value to you (as opposed to the value you're given others by effectively mentoring them). That is just what I do. However, at the very least I expect you could find good developers, future employees, interesting stories and people to bounce off ideas and help test your software.
edit; As others have stated: if you are really interested in startups you should probably be at home working. ;)
I've met many fascinating people there, who between them have been through all stages of the startup life cycle and made many new friends.
I am sorry that you got stuck talking to a bore. In my experience people like that have been very much the exception rather than the rule.
Anyway, I value all feedback and want the meetup to be as useful and interesting as possible so if you have suggestions or ideas on how to improve it I would love to hear from you.
I agree it doesn't add a lot of value to the community, though. Speaking for myself, as someone who works at a startup, I was actually busy working last night. Like most nights.
Maybe I got lucky and chatted to the right guys.