The charity model is certainly not without its flaws, but so far it has worked quite well in the UK. Modern helicopters, advanced medical equipment, well-trained staff and an excellent safety record. The alternative is either government operated and be starved of investment in equipment and personnel (just look at all the Defence Reviews) or a fully commercial operation like in the US with a less than stellar safety record.
The charity model is widespread in UK society; there are more than 1,600 cancer charities alone. Of course the excesses need to be tackled, but the current system has forged a strong bond between local community and their air ambulance, which simply would not exist if the service was government funded / government operated.