Simon Pegg

Neil Clarke

Kate Beckinsale

Catherine

Robin Williams

Dennis the Dog (voice)

Rob Riggle

Grant

Sanjeev Bhaskar

Ray

Joanna Lumley

Fenella

Marianne Oldham

Rosie

Eddie Izzard

Headmaster

Robert Bathurst

James Cleverill

John Cleese

Chief Alien (voice)

Judy Loe

Canteen Lady

Professor Brian Cox

Self

Meera Syal

Fiona

Emma Pierson

Miss Pringle

David Annen

Newsreader

Mairi McHaffie

Newsreader

Gavin Scott

Newsreader

Terry Gilliam

Nasty Alien (voice)

Eric Idle

Salubrious Gat (voice)

Terry Jones

Scientist Alien (voice) / Van Driver

Michael Palin

Kindly Alien (voice)

Alexa Davies

Sandwich Girl

Neville Phillips

Author

Ronan Summers

Security Man