As someone who's been through the process before, it is vital if you can make it down to London at least a couple of times before your sitcom is performed to attend the rehearsals.
It is amazing how a director and actors can misconstrue the nuances of your work. But more importantly, the director or actor might make suggestions which sharpen up the dialogue of your script.
You should also try to get down to see your script performed on the night and to see the other finalists work. It is an invaluable learning experience (and a good night including subsequent piss up).
If you're worried about finances, then don't worry, I'm sure you can sell illegal drugs to children in your town quite easily. Just don't get caught by the police, as it can be embarrassing.
Good luck!