Do emergency services helicopters fly VFR when leaving base? I've recently been watching an interesting documentary on helicopter paramedic teams. As you all probably know, their job consists of a lot of sitting around doing nothing before a scramble and then a short (or not-so-short) flight out to the middle of nowhere and back again. On the (UK) documentaries I've seen, the aircraft are usually based at reasonably large commercial airports, physically located close to the fixed-wing runways. 

In order to find the guy lying injured in the field and help him out of asystole, they have to rapidly launch from a presumably busy airspace, and get there ten minutes ago. Do airports have procedures that allow them to always assume an "exit route" is always okay, under some ceiling? Do they have to get take off clearance as other private aircraft would? I understand that they get given priority ATC clearance when in controlled airspace, but quite often their target call-to-takeoff times are less than two minutes, and it seems unlikely that the nearby 777 on FA would be done that quickly.

What happens if it's not possible to be VFR entirely on route? I have seen cases reported where difficulties are encountered due to fading light halfway through treating a patient -- 
meaning that the pilot is worried he can't take off again. It seems rather odd that, if a pilot was able to land in a field comfortably with good visibility, he can't ask for a route to be cleared, file a flight plan, and go to hospital IFR. 

For that matter, how do police helicopters chase the bad guys at night in zero visibility? 

Disclaimer: I've never set foot in a helicopter in my life, but have a single-digit number of hours in light aircraft -- sick people are more my forte.
If the helicopter is operating at an airport, it needs clearance like a regular flight, otherwise it would be a very dangerous situation for the helicopter and any other traffic in the area. Usually the ATC will give priority to the emergency helicopter (as much as possible), so it will leave the area without delay.

Regarding the weather conditions, there are two aspects that need to be considered:


The regulations allow emergency helicopters to fly VFR below VMC minima (this is true in Europe, I have no idea about FAA). So, an emergency helicopter, can continue VFR with visibility down to 1km, or even 800 meters for short periods of time.
If the helicopter is IFR equipped and the pilot(s) IFR rated, it can switch to IFR if necessary. On the way to the accident site this might not be very useful if the weather will not allow safe descent and switch to VFR, but on the way back to the airport/hospital, it can be very useful, provided the landing site has some kind of IFR procedure. Again, ATC will usually offer full cooperation in all this switching of plans and flight rules.