The question is, would the aircraft go and work without the police observer?
If the answer is no, then you have reasons to suggest that the observers are crew.
Do they perform functions essential to the operation of the aircraft?
If the answer is yes, the observers can be argued to be crew.
Is there UK aviation legislation recognising them as crew? I am guessing not. The situation is similar to here in Aus. They are not licensed, but are really the whole reason why the operation is using a helicopter. Aerial suppourt to police operations.
As far as I am concerned, anyone who is fitted out like a driver, with helmet, flying suit and safety training around helicopters essential to safe operation and operational purposes is crew.
Thats my two cents