In fairness you chose to fix the mortgage and no one held a gun to your head, you signed the contract, you gambled that rates would continue to rise but they did not. If rates had of risen and now your paying less than the standard variable, would you complain?
The only advice i have is to contact the bank and request a copy of the original documentation that states your breakage fee, its a long shot but banks make mistakes all the time and they made have made a mistake in yours.
Also, to the person who says they will "exaggerate my 'changed circumstances'", i'd be very careful with what you say. At the moment your paying your mortgage every month (i assume), thats all the bank cares about, you can cry poor mouth all you want but if your paying they wont care. But if you lie on a communication with a financial institution it can be taken as an attempt to fraud.