Buying kit? I'm sure it's one reason why, but there are many others why most air ambulances prefer to remain charities.
Just imagine that tomorrow the NHS announced that all air ambulances were henceforth being run with NHS funding - much celebration by the public that finally a wrong has been righted. A year later a highly paid consultant on a performance-related bonus conducts a 'review' of air ambulance funding with a view to reducing costs. His conclusion is that there is absolutely no proof that air ambulances save lives (it's true. There is no proof) and that the NHS can save £xxx billion by reducing the number of cabs to 3 nationally.
Far fetched? Or an inevitable outcome. I know which I believe.
Keep the charitable status.