Well,
I have been in cotact with SHINE (the production company that makes Masterchef).
They inform me that there is no actual prize,just the honour of being the winner of Masterchef.
However,many chefs/restaurants WILL be after the winner to train with them,and to eventaully work with them;of course many winners in the past have used this as a stepping stone to opening their own restaurant.
This:~
http://www.thewildgarlic.co.uk/
is the restaurant website of Matt Folas who won Masterchef 2009.