4G? Don't hold your breath
Airwave is an encrypted service with over the air re-keying (changing the encryption key)
This is a trunked service, is both secure and resilient and it allows multiple agencies integrated communications through a nationwide network. It is a secure digital, encrypted network and can be used for voice and data transmission. The radios have a large number of talkgroups (closed groups where you can talk to others in your own talkgroup) and they need programming to give you the Talkgroups you need.
The radios are nothing like mobile phones, they are on for extended shifts and current mobile phones don't achieve that kind of battery life. They are in permanent listen mode and operate on press to talk (like a walkie talkie). They also have an emergency button.
So it will all need custom design. The radios are ruggedised and a search for TETRA handsets will give an idea of how they look and function.
It might be possible to use 4G as the radio network bearer, but they would need to replicate how this currently works (trunking and talkgroups) and everything else has to be redesigned or replaced. Costs= astronomical, say personal radios are say £700; vehicle radios ~ £2000. The radios are personal issue so a force or division of 2000 people = £1,400,000; say they have 300 vehicles = £600,000 total = £2million. (I have no real idea of the current costs)
Also every control room has to be upgraded. Those rooms that are using proprietary hardware can only go to their supplier for the kit they need (or completely change it out). Those that are using off the shelf kit, the interface and software will need changing.
For all of this, there will be tenders and contracts, plus a changeover of the live and working service.
Where are the Emergency Services going to magic up that kind of money?