Hmmmm…I know no celebs. I have had chance “conversations” with 3 1/2 that I can think of (but there’s a catch with 2 of them). Jim Moir (aka Vic Reeves) and his wife Nancy, Ronnie Wood and Richard Branson.
The proviso is that the conversation with Ronnie Wood was in a queue for the loo at a party and the conversation went along the lines of:
RW: “**** me, a bloody queue for a ****?!”
ME: “Yeah”
RW: “I’m goin’ outside”
ME: “Yeah”
…and with Richard Branson, passed him on the stairs at Virgin records
ME “Hi Richard”
RB “Hi. How Are You?”
ME: “Fine Thanks”
RB: “Good”
I’m not exactly trained to be “A list” fodder as you can see 
Oh yeah…nearly forgot… the boy-band “Blue” ran over my foot in a London backstreet while escaping from a crowd of screaming girlies in their 4×4 once.