I don't think anyone is advocating that Mann would or should be a county of England, it's more a case of an `independent' body that has experience in inspecting and making recommendations over safety of public and heritage transport systems. I'm sure any of the home nations will be equally as good as an 'English' or 'Westminster' inspectorate.
If there's politics involved then it seems as if it's very much within the executive. Mitigating risk can only be a good thing, shirley?