Had another quick look at this. There actually doesn't seem to be a law saying that employees have to go home or do not come to work when there is a T8 or black rainstorm.
What there is, is a general law saying that employer's have to take account their employee's health and safety.
Further, there is a code of conduct issued by the government as a best practice guide on what do do under a T8 or a black rainstorm.