I've had the (dis)pleasure of meeting a surprisingly high number of people who are now movers and shakers in Labour. I got into an argument with Seamus Milne about something at a book signing when he was just a lowly journalist, and we chatted for about 15 minutes afterwards. I've met Owen Jones too, but more fleetingly. Both of them were quite disappointing experiences.
I met and briefly interviewed Vadim Gagloev, who until recently was head of the Russian state circus organisation. I also interviewed a magician who teaches in Moscow, a reasonably famous Russian tapdancer, and some clowns who are well-known over there. There's probably legal/data protection implications for naming them on here, so I won't risk it. None of these people really count as properly "famous" I suppose, but it was a privilege to meet them and speak to them properly (at least to the best of my ability!) and they were experiences I treasure.
As for properly "famous", my best claim is probably a 5-minute conversation I had with Ian McKellen when I was about 17. I had only recently come out at the time, was absolutely terrified (and probably extremely annoying), but he was very gracious and supportive to me. I have his autograph tucked away somewhere.