As I understand it, there will be picket lines at Royal Mail sorting offices on Thursday and delivery offices on Friday. The managers are actively scabbing; breaking health and safety practives such as carrying two bags at a time and using their own vehicles. Workers at other communications firms, such as UPS, are scabbing as well and this is so far going largely unchallenged. An effective strike should be made industry wide using flying pickets.
See the link for detailed information about this dispute.