Don't pay that price for sure, shop around.. I would take it to another MOT station for a second opinion, tell them it failed the test and see what their solution might be. Good testers can tell a lot from the way the brakes perform on the rollers. Unless your brakes have been neglected for a long time I doubt they are going to be seized to any degree that will cause that amount of imbalance. If they are seized or binding in any way I would take the opportunity (funds permitting) to overhaul the front brakes with new pads and discs anyway. Failing that, do as said and give everything a good clean and make sure they free to move.
I know others don't agree but in weather conditions like we have now the filth off the roads can play havoc with brakes. The undersides of cars around here are covered in about 6mm of black gunge! Lots of it goes onto the brakes as you drive, meaning the pads have to work that much harder to do their job. It could even be that today your brakes will give different readings to those before because of it.