"But a big negative for the current providers. It's now easier for some upstart to move into the market as they don't need to build a full network all at once."
Not really: first, we have plenty of upstart operators with no network at all, some even owned in whole or part by the main networks themselves (Giffgaff by O2, Tesco Mobile part-owned by O2). Second, that roaming has been an option for years: because of their "significant market power", both O2 and Vodafone were already required to provide roaming services to the other operators if asked. When Three were getting started, though, they contracted with both Orange and T-Mobile for this instead.
So, all five (now four) operators have already either been required or have chosen to participate in similar arrangements for years - it just hasn't been much use.
I happen to have a SIM card which is already capable of domestic roaming (I can choose whether to use O2, Vodafone or EE - Three should also be an option, but apparently doesn't work right now) - but it turns out (perhaps thanks to O2 and Vodafone having merged network operations under "Cornerstone" anyway) that in almost every single case I've had a poor signal from one network, it's been much the same from the others anyway.
TL;DR: Government making a big fuss about "forcing" the networks to offer stuff that was already available anyway and actually offers negligible benefits. Rather like Perry's efforts to ram filtering down everyone's throats...
Biting the hand that feeds IT
