Not sure why you get banned. I always post HNLondon meetup announcements while it is morning in UK and SF is asleep. At that time there is less competition for the front page and it's possible to get there with just a few upvotes. Once SF starts to wake up we inevitably get swept away.

HN London almost always gets front paged. Maybe the account that submits the SFHN meetup is blacklisted for some reason? Probably a question for the admins.

HNLondon has 5000+ members while it looks like you're an order of magnitude smaller, hence the HNLondon post is likely to be upvoted by a much wider range of people and thus less likely to be hit by the vote-rigging detector.

Although looking at your past submissions they don't appear to have been blacklisted.

Probably triggering voting ring detectors. If you email info@ycombinator.com I'm sure they'll be happy to tell you what's up.