As the Devil and Tim Worstall point out, if Sir Fred Goodwin's pension was part of an agreement reached between him and RBS as he exited the company, there's really no grounds in law for confiscating his pension money if he's not willing to give it up voluntarily. Sir Fred claims that he has already made a number of gestures in terms of his entitlements from RBS, and that the pension relates not just to his time in RBS but also his previous employments. Sir Fred also makes the claim that his pension arrangements "have not fundamentally altered" from when he joined RBS in 1998.
During his time in RBS, Mr Fred Goodwin became Sir Fred, due partly to the intervention of the then Chancellor. The same Chancellor whose regulatory system completely failed to prevent the collapse and (partial) nationalisation of several banks. Perhaps Sir Fred should consider this - offering to take the same percentage cut in his pension as the then Chancellor, given that they both messed up on the job.
I wonder how well that would play with the current resident of Number 10 Downing Street.