At startups? Sure. Several stints at non-startups? Well, how much of that time was spent learning the domain? Is that knowledge transferrable (probably not because of non-competition clause)? Why were you not happy at the previous employer?

I am not saying any of these don't have valid answers. What I am saying is that we would prefer juniors that are commited and do the hard work when the work gets hard. And, at least where I work now, this gets recognized, and they become seniors in time.

London has not been part of Europe for a decade now. They locked themselves out in a bout of insanity

Indeed it used to be the norm since basically everyone worth hiring was a contractor prior to the IR35 debacle.