A court issued that ruling. Not an administrative agency. Pretty big difference.
Ofcourse, everybody ( well, outside the UK ) are OK if UK orders UK ISPs blocks sites for their customers.
Another advantage as the other reply has mentioned is that courts have broad authority but must narrow the effect of their rulings to the minimum necessary to address the suit. In this case it would certainly lead to 4Chan being blocked by UK ISPs by order of a UK court. I think even 4Chan would be fine with that.