EMSR22,
For many years NVG was not permitted by the UK CAA for police flying. I was not directly involved, but I believe that failure of the NVG system at critical points in flight was a major issue for the CAA. They have now been reassured, but I believe that they have particular requirements relating to power supplies etc.
Initially, NVG was permitted above a minimum height. After a period of operation, the approval to use NVG then gets extended to cover landing and take-off.
If pure HEMS at night starts up in the UK, it will be interesting to see whether the CAA is able to impose "extra" requirements over and above those to be mandated by EASA. I thought the whole point of THE DIRECTIVE was to have common standards, but we will have to wait and see.