Why Just 4G???
Given that you can get mobiles off the shelf that have three 2-4g modems built in why can't you just use something like that so each unit has a live connection to the 3 physical mobile networks in the UK and then just route's over the strongest available signal.
Couple that with a local area mesh network based on IEEE 802.11s to offload the bulk of local traffic and I really can't see why this is so expensive??
I understand that 4g is the first generation of mobile network that supports emergency traffic priority but it seems to be a very crap trade-off.
You could take it even further by placing high power repeaters into the vehicles.