Whilst there is nothing to prevent Westminster passing laws which apply “henceforth and for ever” – they have, after all, had a few centuries of practice at it – making even such a sweeping phrase stick is another matter entirely, given the constitutional convention that no parliament can ever bind its successor. Whatever can be done by a parliament which believes itself to have absolute sovereignty can always be undone on the same basis. I wouldn’t be at all surprised to find that, in legal terms, the UK Parliament has the theoretical power to revoke Australian independence (although such a move might be rejected on the minor issue of being utterly impossible to enforce).
Would it be different if the UK had a formal written constitution? At one level, maybe not; a constitution brought into existence by a parliament which believes that it has absolute sovereignty can be abolished or changed by the same route, although it might be a little harder politically. They key element which needs to be changed is that core belief, that underlying principle, that the UK Parliament has absolute sovereignty. Changing that element demands that we get rid of the constitutional fiction that power is something which comes from the top – in the case of the UK, from God via the monarch to parliament.
For most practical political purposes in terms of the sort of legislation passed by political parties which have chosen to limit their ambition largely to what is achievable by minor incremental change, the question of the fundamental underpinnings of the power structures in the UK is irrelevant. A constitutional monarch does as (s)he is told, and there are more important priorities than abolition. But when it comes to some constitutional issues, challenging the basis on which that power exists is an essential pre-requisite. And the ‘permanence’ of the Scottish Parliament or Welsh Assembly is such an issue.
Only in a state where power is formally recognised as belonging to the people, on a bottom-up rather than top-down basis, can the rights of parliament to legislate be properly and fully limited. It’s hard to see how that can really be done without abolishing the rights of kings – something which the establishment parties are unlikely to tackle. And that, I suspect, is at the heart of the reason why the UK doesn’t have, and is unlikely to have any time soon, a written constitution. It raises too many difficult questions which they’d have to answer.
Back to Smith, and I expect that the parties will find some convenient form of words to pretend that they’re making it impossible to abolish the parliament. But the only real guarantee that the Scots (or the Welsh) can have is by making it impossible in practical terms rather than in legal terms. And I’m fairly certain that we’ve already reached that point, even if Westminster hasn’t yet fully realised that fact.