Superficially, it's a terribly attractive idea. The question is whether any network would be the first one to put a mast in a previously uncovered area on the grounds that they'd have all the capital cost with none of the benefit, resulting in the law of unintended consequences and poorer rather than better coverage. On the other hand, there are virtual networks that seem to co-exist so, if there's a payment model that works for them (and the main networks), maybe roaming could as well.
The other downside might be that, instead of multiple masts covering an area, there'd be only one, meaning that, in the event of failure, all the networks would be down instead of just one. If you're on that one, of course, it's always 100% of your network. There'd also be the issue of congestion. If all the networks were sharing a mast, it would, by implication, have to have more bandwidth (if that's not the right term, I hope you know what I mean) or there'd be a lot of dropped calls and slow data traffic.
Isn't the whole situation the result of Ofcom insisting on networks reaching a percentage of the population, rather than the country? To to this on the timescale required meant concentrating sites on areas of heavy population - ie towns and cities. (I believe they've changed that recently, so perhaps some of the netwrks might actually co-operate anyway??) It's often argued that rural areas are sparsely populated and therefore uneconomic (and that's where voluntary internal roaming might be attractive), but most of these areas also have through traffic, which consists of people who live in urban areas and therefore are economic. Are networks currently allowed to share masts?
It sounds like one of those answers that's so easy you really ought to be asking yourself, "have I understood the question?"
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