I worked in London for many years, so saw quite a few celebs.
Loveliest:
Boris Johnson - used to wave & say "good morning" to me every day as he rode by on his bike
Victoria Beckham - about 15 years ago she was in the queue behind me in a supermarket near her home in Herts. She was lovely (& is very pretty in real life). Didn't want any special treatment & happily waited her turn.
Hugh Grant - was very sweet when I accidently stood on his foot & then sort of screamed in his face that he was Hugh Grant 
Rhod Gilbert - was very polite, waved and said hello when I was staring at him trying to figure out why he looked familiar
Damien Lewis - smiled and said hello to everyone as he walked through Covent Garden
Guy Ritchie - said hello to everyone as he was walking towards a street where Madonna was filming
David Beckham - sent signed photo autographs to my nephews when I asked someone who knew his mum
Peter crouch - signed autographs & chatted to everyone on the train for about half an hour, even though he looked tired & had his family with him
Worst celebrities
Madonna - very rude to everyone around her! Unofficially closed down a street where all the food stores / restaurants were one lunchtime so she could get the perfect shot. After 20 mins people started to get annoyed and wanted to be allowed past and she threw a massive diva strop. I wouldn't mind, but they weren't even filming at the time!
Jamie Cullum - actually used the phrase "yes, it really is me" when I was stood behind him wondering why some idiot was just stood there blocking the queue at Heathrow very early one morning.
Denise Welch - was drunk & attention seeking whilst waiting noisily with her boyfriend in the lounge at Euston (it was only about 6pm)