This is from the BBC's comedy section, they ideally want fully written scripts not just ideas.
http://www.bbc.co.uk/comedy/feedback/faq_writi ng.shtml
Where can I send my comedy script or idea?
We are sent a lot of ideas for shows such as Little Britain. While we love to read your ideas, most writers will not accept submissions from members of the public, as they prefer to write their own material. However, if you have written a complete original script you can send it to the BBC via the Writer's Room website.
I've written an original comedy sketch. Where can I send it?
Generally you cannot submit individual sketches for commissioning. However, some radio comedy programmes encourage submissions from new writers - check the Writersoom Opportunity page to see if any programmes are currently accepting material.
C4 and Five are just comissioning channels and so don't produce their own shows, they get indies to do that. I think the BBC is the place to start.