I'm guessing, but should imagine they get wind of exit polls on the fly and adjust their odds accordingly? They're not allowed to publish what they receive but you can't prevent them adjusting their odds. Either that or they are simply reacting to the markets if indeed they are bouncing (I don't know whether or not that is true).
Or maybe we've all been waiting to find out what Bobby Zamora thinks. Happily, he's one of the good guys.
A number of hedge funds have paid the polling companies to conduct private exit polls. This is allowed under current legislation. It will cost them £5-10,000 for such an exercise.
They are using this information in the currency exchange markets, betting millions on the outcome before the result is known for certain. This will of course move the markets as they buy or sell.
Sterling is up by 1.14% this morning already, which suggests that............