Causing an ac to go-around at 600ft isn't ideal so why wasn't the decision made earlier? Did the police helicopter give late notice of his intentions? Is it a case of "comercial pressures" that LHR press-on until the last moment before sending traffic around? (not a dig, genuine curiosity!)
Category of flight is to allow aviation on high priority tasks to be able to execute those tasks. If an ac is involved in safety of life/security then surely he has to be given priority over aviation involved in commercial enterprise. That said, carte blanche issue of CAT B to police heli's has to be wrong. Traffic monitoring or routine security checks do not warrant category B status IMHO.