From the DTI Guidelines and which must be printed in full on the ballot paper:
“If you take part in a strike or other industrial action, you may be in breach of your contract of employment. However, if you are dismissed for taking part in strike or other industrial action which is called officially and is otherwise lawful, the dismissal will be unfair if it takes place fewer than twelve weeks after you started taking part in the action, and depending on the circumstances may be unfair if it takes place later.”
A company does not have to pay workers who are in breach of contract, which is why some unions pay strike pay. A standard tactic is to suspend the worker pending an investigation for the breach. This is why IA is disproportionately unfair on those who happened to be rostered to work on strike days, even if the strike had 100% support.
As to whether it is illegal to discriminate against those who take strike action I do not believe this is true (although they have some limited protection from dismissal as above). It IS illegal to discriminate on the grounds of race, sex age or religion in terms of pay (but not concessions such as staff travel), but not on other grounds. Even if dismissal is illegal, lack of promotion, demotion and unfavourable rostering certainly isn't!
It would be interesting to know the legal side of suspension without pay - of course even with basic pay the drop in allowances sat at home would be substantial.