In particular, it is difficult to envisage any sanctions for breaching the act which would outweigh the political stakes involved if a government thinks that holding information back until after an election will assist it at the polls.
She's right and wrong here. Minor sanctions won't influence Ministers, who we already know put their careers before the law. But they will influence the professional public servants who actually do the work. Criminal penalties will give those public servants a reason to resist Ministerial lawbreaking, give them absolute protection from employment sanctions for doign so (because you can't fire someone for refusing to break the law for you), and if Ministers say "I'll handle it", create a clear paper trail which will assist in their prosecution. And if the maximum penalty is two years jail, triggering automatic removal from Parliament on conviction, then that may give even Ministers pause for thought - if only because they won't want to have their future political career reliant on the silence of their past Ministerial advisers.
The trick is persuading Ministers to vote to stick themselves in jail. And that is going to be very, very difficult.