Simon,
In answer to your questions.
1. How did you know they were Police helicopters? Was it the paint scheme or the aircraft type? When the Police fleet was upgraded to 135/902 all of the AS355s were sold to other operators. Most of them left them in the same colour scheme to save expense plus it is a recognised high visibility scheme for low flying aircraft. A lot of these aircraft are used for utility and aerial work like airborne rebroadcast.........
Could this be what you saw?
2. Go to any big event that requires on course cameras, spread over a large area and with some that need to move with the action then you will need to get their broadcast signal to a central production vehicle and then a "satellite van" for onward broadcast to the network. The cameraman standing on the back of the motorbike filming the runners in amost the buildings of the city with no line of site to the satellite van. He will transmit to a rebroadcast helicopter who will have line of site with both camera and van. The rebroadcast aircraft are easy to spot as they usually sit in a higher altitude in the hover or flying small orbits. Camera aircraft tend to be lower and follow the action. They may have had two rebroadcast aircraft as there was a lot of cameras to service or they may have been doing a running "relief in place" to give continuous coverage.
Hope that helps?
FNW