A helicopter has never treated a patient. It carries people and equipment that may treat patients. The amount of equipment in a HEMS aircraft is greater than in a SAR helicopter.
Horses for courses.
In fact with a major incident HEMS role is primarily to deliver medical staff if they cannot travel by road - don't know if that applies here - and possibly to extract specific patients such as head injured patients who need to go to a regional neurosurgical centre as opposed to the receiving centre.
Using multiple receiving hospitals sounds good but with this number of casualties is often unnecessary and indeed disruptive to the NHS such that it is contraindicated.
I suspect the controllers played safe ( possibly over reacted
) at the initial call as the number of casualties and the injuries didn't justify this response. Of course HEMS is free to the ambulance services