Interesting posts, JayPee28bpr.
It is possible that Unite's lack of action on pursuing the staff travel issue through Europe is a strategic decision on their part. The issue of removing perks from striking workers is a significant unresolved point of law, and one which potentially affects every single union member in the country. As a composite union, Unite has many members who would be affected by the outcome of the court case, not just members of BASSA.
Very few people outside BASSA seem to view the current industrial action as objectively justified (i.e. whilst of course they have the right to strike, it seems a disproportionate reaction to the issues in dispute). As you have identified, the court would be required to balance the competing rights involved, and Unite may well therefore be waiting for a case where the balance is more in their favour before pursuing the point. If they then manage to get a favourable ruling in another case, it would put them in a stronger position to argue for the reinstatement of staff travel in the BASSA case. But, more importantly for Unite, it would mean that they had best served the interests of all of their membership rather than one small part.