Maybe this was just an attempt to distract from five years of Tory/Lib Dem slashing of police budgets or maybe Damian actually believes that this is a remotely workable idea.
You don't even need to be an expert to dismantle his argument.
Damian's idea suggests that one vehicle could be designed and purchased that would carry a crew of trained firefighters, two or three paramedics and a number of traffic officers to secure the scene, as well as the range of kit required to manage the scene, extract the injured and stabilise them. @martinwoods suggested this.... (HT also to @SgtTCS )
There's a reason that this hasn't happened. It is that there are different vehicles because they all have different jobs and equipment, as well as fulfilling other roles independently. The fire service will respond to a house fire that will not require police attendance, the overstretched roads policing units will be out and about tackling crime involving vehicles and enforcing on traffic offences while the ambulance service will spend their time waiting around hospitals for the emergency department to take their patients before they head off to the next job. They cannot spend their time waiting around for an incident that requires all their specialist skills.
They will have different times and jobs at the scene - the fire service will attend to extricate the injured and make the scene safe, but can then leave. The paramedics will respond, stabilise the patients and then leave to take them to hospital, while the police can be at the scene for hours, securing it for evidence purposes and investigating the cause.
The reason that it hasn't happened, Damian. They gave different, specialist vehicles because they are different specialists. Even for a former Tory minister, that was a spectacularly stupid idea. But when has that ever stopped it becoming policy?