Staff travel as a contractual benefit
It is my understanding from posts some time back on the CC thread that staff travel was specifically negotiated by the Union as a non-contractual benefit, in order to take advantage of the favourable tax treatment.
Just because the benefit is treated as non-contractual for tax purposes, it doesn't follow that it must be protected for employment law purposes - the tests are not the same. So it is, just, possible that a judge could find that staff travel is protected under employment law for strikers, but the tax-free status could remain.
However...there appear to be several legitimate legal arguments either way as to whether it is a protected benefit. So it could go either way. In weighing up those arguments, the tax status of the benefit is not strictly relevant to whether it is a protected benefit. Judges are only human, though. I think that a judge is likely to harbour a feeling of 'you can't have your cake and eat it' , which means that the union are going to face an uphill struggle to win the argument in court.