I predict that SIMs will allow access to all technologies, and that support will instead depend on handset for the time being. That's only a personal prediction, though.

I think there's the potential to see networks granting LTE-only access on consumer tariffs once coverage is sufficient. But most if not all GSM networks in the UK will need to maintain GSM capabilities for some time yet if only to support deployed M2M solutions (things like chip-and-pin machines are still being sold on 2G now), so I'm not sure they'll be fussed about it.

I'm not sure what the issue was when 3G launched. Part of it seemed to be about supplying more capable SIM cards (perhaps there were security implications), but part of it also seemed to be about restricting access to the new network, trying to retain some level of control initially until all the kinks were worked out.