lady_p_gold... that's not entirely true - the decision to use the audible and visual warning systems, and the exemptions from the law, is the decison of, and must be justified by, the driver, regardless of the grading of the call.
An example of a non-emergency call would be trying to pull over a vehicle - not something that the control room would allocate, or an urgent assistance which wouldn't be an emergency call but (in the Met at least) would be graded as "Police Generated". Police Generated would also be used for a non-priority statement.
Every accident where a police car was involved has to be fully investigated, even if it wouldn't even be reported if it had simply been two cars driven by members of the public.
Also most police cars now have data recorders in them which are downloaded and will be checked by the investigators - they will see the speed and way (including lights etc) that the vehicle was being driven.