Ozzy
The fact that an act is illegal does not necessarily mean that you can be prosecuted by the Crown.
I am no legal expert, but my understanding is that you will not be prosecuted for undertaking a wildcat strike. However, a union that advocates or organises such a strike can be, but the penalty would be a fine / freezing of assets. Even this is very rare.
An individual does effectively sacrifice their right to a claim of unfair dismissal if they are involved in an illegal strike though.