A good site to be free of the fee here
I've had them round a few times, each time I politely say "no thanks" and shut the door, no contact no contract!
If they feel threatened they can be ask for police backup, the police are only there to prevent a breach of the peace, they don't have a right of access, as long as your polite they can't do anything, the onus is on them to prove your breaking the law, so never admit anything.