HEMS Chap yes but in reviewing the state of the nation it would have been incorrect of me to say that HEMS are 24/7 night flying. Daylight only is the broad picture, night capability is where some are moving towards.
They are all different, shades of absolutely nothing to operating. Only the police [still not 100% but the intent is there when the held over units like Lancashire close] and SAR can be declared as 24/7 night flying.
I am sorry but the majority of UK HEMS are not yet night flyers. Extending hours or return flights into the dark is not in any way a solid commitment to night flying. Even those with proper night flying on their agenda are still talking of close down around midnight, maybe 2am. Yes some do more than others but many have no intention of doing 'real' 24/7 at the moment simply because the demand is currently seen as too patchy or there is simply nowhere to fly to..... sort of pretty useful to have a 24/7 helipad at a hospital if you declare a capability!
As you say.... Each does what they think is required and affordable for their own "patch".... but NPAS and SAR is a national commitment.... you still need insider knowledge to know where it is safe to break your leg at night...
Only when we are anywhere near the point where you can stand anywhere in the UK at any time of day and demand a HEMS air ambulance is there a night service. At the moment the odds are well stacked that either the police or SAR will turn up. As long as the weather and serviceability are kind of course.....
I have little doubt that a handful of UK HEMS may decide to go real 24/7 in a year or three but near future [say 2017] its still down to the police and UK SAR to provide that service for the foreseeable future.