Starting pay will vary depending whether you are directly employed by the police, or through a contractor like PremiAir or PAS(Police Aviation Services). I think that there are about half a dozen Forces who employ direct.
The contractors pay around £39-41000, with the direct employed getting maybe 4000 more. This is all for the Police. Not sure what the EMS guys get paid, but I wouldn't have thought that it's much different.
PremiAir(used to be Operational Support Services/McAlpine Aviation services) are currently advertising for a couple of pilots. You can contact the Chief pilot on 01895 830930.
Your civilian background would not be a problem.