Thing is these days it's so easy to write jokes, find some's address and send them via email that the market is flooded. It's very hard to stand out.
And in my experience the majority of all but the most succesful comics would rather sell you gags or teach you how to make them.
The only real route is to either get noticed or get contacts.
You can of course just e-mail a comic and hope you're good enough, but that's damned tough.
More likely try,
Posting on sites like this
Posting on Facebook, Twitter etc
Building your profile through writing for sketch shows
Do standup loads
Joining groups like London comedy writers.
But be prepared to be going for a long time before you get surprised by your success