So far as I'm aware, this would be the first move to introduce plain packaging as a rule to cereal packets. As such I don't think there's going to be any direct evidence to show how useful or not it would be. (If anyone does know of such a study I'd be interested to see it.)
So I thought it best to mention cigarette packaging because that's a reasonably comparable example: an unhealthy product where advertising has been shown to increase uptake rates -- and, correspondingly, changing the packaging to remove brand markers and replace them with health warnings and pictures has been shown to have a small but non-zero impact on usage.
Would it be the same with cereal? Who knows? For the time being, I think Labour's proposal is limited to selling cereal in bland boxes rather than ones with pictures of fat children. But, since advertising works, then action to block advertising of unhealthy sugar-laden cereal products is bound to have some impact.
If that is the only part of Labour's policy on promoting healthier eating habits then it's merely a token gesture. If it is part of a wider package, including, say, action to try and force a reduction in sugar content, various taxes, reductions in the price of healthier food, etc, then it could be worth trying.