In the UK the NAA means the CAA.
Not quite sure that is strictly true. Apart from 2 (currently) police/HEMS units, it is true that they do not operate at night to off airfield landings.(The Scots do essentially inter-prepared site flights at night, which isn't really HEMS, of course).
I believe that JAR-OPS does not prohibit night HEMS. I suspect that the CAA/UK NAA do not exactly prohibit it. They just make their list of conditions and requirements so long, that once faced with them, no UK ordinary HEMS has yet accepted the challenge.......
That is almost a prohibition, but not quite.
Being as risk averse as the CAA is, with the US record or night HEMS, I would imagine they will try and keep thinking of extra requirements, to prevent a rise in UK HEMS accidents. In a way you cannot blame them for that.