I've just got back from a coach trip to the south and to while away the time kept a look out for an Up! of any colour etc. Never spotted a single one. After reading all the comments in some of these forums I suppose the reason is they are all still waiting at the ports in Northern Germany.
I've seen four (1 red seen 4 times now, three whites) but there could be a duplicate in the whites, so maybe 3. I was filling my Up up last night at the local garage, and the attendant said he'd seen a red Up fill up. And he said they were roomy inside. I think he has not read the Which review, but just sat in one instead.