I don't have the numbers in front of me but I think the UK HEMS accident rate is mercifully low, especially when compared to that of the USA. Do you think that there might be a connection between that low accident rate and the lack of night flying? If it ain't broke, don't fix it.
If, as Jayteeto says, the numbers of ex-military pilots flying HEMS in the UK is much lower than the past, the risk involved by introducing NVGs into the mix is magnified further. For all the veteran pilots with 500+ hours on NVGs or, as Bap70 described, paramedics with thousands of missions behind them, there'll be just as many who are just finding their feet and feeling pressured to get the job done because the other guys do. Do you think the tasking authority or the management or the fundraisers or the public or the media will know which ones are on duty on any given day? Will they hell.
Do yourselves a favour and stick to daylight. Bats and prats and all that.