When a solicitor from Direct Energy came to my door and told me I was *required* to give him copies of my electric bills, I told him I was going to call the police to check on his solicitor's license. The PD sent officers, and I pretended to be on hold until the officers arrived.
They arrested him. Posted on Facebook for other locals to do the same. Battle WON.
Don't give in to pushy door-to-door sales people - that's all these people are. If you want to take advantage of fixed rates, do your homework (my advice).