Hi,
I'd check the terms and conditions you were sent when you signed up for this tariff, they should confirm the details of a termination fee. I'm not aware of the exact details however lots of suppliers have cancellation fees for Capped or Fixed tariffs.
It does look as though the prices may drop soon as some suppliers have announced decreases but always check first as the rate you fixed or capped at may remain better value than the amount the prices may drop.
If the cancellation fee definitely stands one idea would be to change tariff via a cashback website as this will offset the cost of cancelling this tariff.
Brian
Employee of E.ON