They would class a Chemical incident as a 'major incident' almost regardless of the size because of the possible outcomes.
Not quite. A major incident is defined as an incident dictating that the local hospitals would be receiving patients beyond the level that is normal for them. There are dozens of chemical incidents across the UK everyday that are unreported and that are not declared as majors. Nothing more than press spin.
A two car collision could potentially produce fourteen patients arriving at ED together, but as with chemical incidents a quick look first to see what's what is the way to do it.