SASless,
The police units that are HEMS approved carry pretty much the same gear as a pure HEMS aircraft (including a paramedic).
The non-HEMS police heles carry a variety of medical kit, sometimes including things like defibrillators, but not usually stuff like oxygen. They do not carry paramedically trained crew, so there is little point in carrying that equipment. When they are deployed to a CASEVAC, the normal SOP is to dump one of the observers at one point or another. They pick up a paramedic with the casualty, who brings what he/she feels is necessary from their land ambulance. I don't think anyone is saying that a CASEVAC police helicopter is in any way a repalcement for a HEMS machine. It is used as a last ditch option. Sometime there is no HEMS or to supplement the HEMS provision. However, CASEVAC can often make a big difference to outcome.
These days in the UK CASEVAC is probably more often than not at night. As has been stated previously, our UK regulations do not, in practice, allow HEMS to do ad-hoc landings at night. Whatever the reasons, no non-police UK HEMS is done at night at present.
In Scotland there is a rather different system from England and Wales. Different set of politicians, so they buy an air ambulance service includng helicopters for the Health Service. The heles operate 24 hours, but I believe that they do no HEMS at night, just transfer type flights.
As Paracab suggests, the distorted world in which UK hospitals operate produces some strange results. As things stand if you convert a hospital helipad to a car park, you not only generate revenue from the car parking, but you PREVENT very seriously injured (i.e. expensive) patiemts from draining the respurces of your hospital by becoming your responsibility when they arrive by helicopter
I imagine that other systems like the US have different pressures.