Correction.... but still a bad idea.
If his problem on the Tunbridge Wells train ride was network capacity, then roaming to another operator probably WOULD resolve the issue; but he'd have to disconnect and re-register, which would be "a lot of annoying bother".
Certainly, the load on the HLRs and also SDCCH (from memory - the GSM RF channel used for such signalling) from people registering on whichever service is stronger as they come out of the tube/tunnel/mountain pass, and then re-registering on their own network seconds later, would be huge; particularly in dense areas like London, and this proposal is impractical because of it. Also, imagine how many people would happily launch into a call "without realising [they] were roaming", and then complain about their huge bill afterwards. Lots.
You can also make emergency calls on any network already, regardless of subscription, so not a valid argument either.
Still, it's nice to see an MP with the public's interests at heart, for a change.
This Dude, because he also looks like a nice man.