Having previously worked for the power companies, i would like nothing more than to join you in calling them evil. However, in this case i can't.
The entire reason the power companies tend to over estimate your direct debits is not, as you claim, a secret cabal of power companies conspiring to rid you of extra money.
The actual reason, is that the power companies are bound by the regulators to have a moral responsibility to their customers. This means that they have to prevent all means of knowingly allowing someone to fall into debt with their bills.
The problem here is actually that, as you know, people use more in the winter than they do in the summer. Therefore, your direct debit has to be based on an average years consumption, rather than a few months. This is why your DD is recalculated every six months, and takes your previous annual consumption into account. When you couple that with fluctuations in energy prices, then your DD is at best an educated guess.
You can of course help the situation by supplying monthly reads to the power companies. The system they have is far from perfect, however it is not helped when all they have to go on is estimated readings.
It should be noted that some power companies are now offering to pay you back £1 of interest for every £3 that they have in credit balance for you. So it doesn't seem so likely that they're trying to fund their companies by deliberately taking extra money now, does it?