In theory some £150m of public money has been given to Arqiva to greatly improve mobile coverage in the rural parts of the UK and while there was some news on the first mast back in 2013, the news output that we've seen minimal announcements since then. Fast forward 9 months and say hello to our new Culture Secretary Sajid Javid who is keen to back the idea of national roaming, i.e. if your phone cannot connect to your existing provider it will piggyback on which of the other networks it can see.
So this begs the question has the Mobile Infrastructure Project failed, or like so many political ideas this is one to solve a specific problem and ensure people are seen to be doing something, i.e. avoid one MP needing to squat against a sink to get a signal or the Prime Minister complaining about reception in his constituency. Though he should be thankful that unlike some previous Prime Minister's he does not have to keep loose change in the car for contacting Downing Street from a payphone and the cabinet in a time of national crisis.
Enabling roaming between operators sounds like an easy solution, but with the operators already sharing the physical masts (just adding their own antenna and renting part of the backhaul) the difference in coverage may just be down to the way that different frequencies propagate through the objects between you and the mast.
The EU has mandated that roaming will be free in the EU from 2016 and some operators are already embracing the idea of allowing people to use their data allowance and call bundles when abroad rather than charging an arm and a leg. There is one danger of enabling it at the national level though, it may reduce the incentive for operators to compete and result in the creation of an infrastructure monopoly, not unlike the one for fixed broadband with the national copper network from Openreach. The end result being that by the time 5G spectrum auctions are held there will be just one bidder and a lot less income to the Government purse.