Department of Health (2008)

In 2008 the UK DoH commissioned a consultation with the results published in a report titled Consultation on The Future of Tobacco Control In it, it reported (with emphasis added):

Research shows that [plain packaging] may reduce the attractiveness of cigarettes and further ‘denormalise’ the use of tobacco products. Studies show that plain packaging reduces the brand appeal of tobacco products, especially among youth, with nearly half of all teenagers believing that plain packaging would result in fewer teenagers starting smoking.[Page 7]

However, later on it goes on to say

3.65 The Department of Health is not aware of any precedent of legislation in any jurisdiction requiring plain packaging of tobacco products.[Page 40]

So there is no research in to what effects, if any, plain packaging elsewhere may have.

3.66 Studies show that plain packaging reduces the brand appeal of tobacco products, especially
among youth. A Canadian study reveals that virtually all 14–17 year olds involved in the
study said that the reason they might start smoking or currently do smoke is to be ‘cool’ or to
‘fit in’. [Page 40]

This is referencing M Goldberg, et al. (1995) which is not a reliable guide as to what effect plain packaging will have.

3.67 The same study also found that: [Page 40]

M Goldberg, et al. (1995) again.

3.69 Plain packaging may also increase the salience of health warnings. Studies show that students
have enhanced ability to recall health warnings on plain packs.[Page 40. My emphasis.]

Or it might not. If they're going to insist on referencing M Goldberg, et al. (1995) for two paragrahs then I'd draw attention to the results of their Recall and and Recognition survey where it was explicitly not shown that plain packaging enables better recall of health warnings.

3.75 As there are no jurisdictions where plain packaging of tobacco products is required, the research evidence into this initiative is speculative, relying on asking people what they might do in a certain situation. The assumption is that changes in the packaging will lead to changes in behaviour. [Page 41. My emphasis]

Indeed it is, though Australia may provide some light on the subject very soon.

3.76 Plain packaging may force tobacco companies to compete on price alone, resulting in cigarettes becoming cheaper. However, if a decrease in price were to follow the introduction of plain packaging, increases in tax on tobacco could counter the effect.

Unintended consequences? And as if there wasn't enough tax on the product in the UK to begin with. The Laffer Curve never seems to bother the government when it comes to taxes for some reason.

3.77 Children may be encouraged to take up smoking if plain packages were introduced, as it could be seen as rebellious. However, the Department of Health is not aware of any research evidence that supports such concerns.

They clearly aren't looking hard enough for teenage rebellion being a cause for taking up smoking:

Ethnic and gender differences in risk factors for smoking onset: Robinson, Leslie A.;Klesges, Robert C. (1997)

In a number of studies, rebellious children have been found to be significantly more likely to smoke

Smoking in Children and Adolescents: R Evans et al. (1979)

The third section reports findings of studies focusing on several psychosocial factors influencing the decision to smoke: [...], siblings who smoke, rebellion against family authority, [...], and perceptions of the dangers of smoking.

Predictors of smoking intentions and smoking status among nonsmoking and smoking adolescents, Vida L. Tyc, Hadley, et al.(2004)

Results: Parental smoking, higher perceived instrumental value, higher risk taking/rebelliousness, higher perceived vulnerability, and older age increased the odds of an adolescent being a smoker.

So it's clearly been researched, and in some cases found to have a possitive correlation with smoking. Not that's it's terribly easy to find this sort of thing these days.

Back to the consultation:

3.79 Some stakeholders have suggested that plain packaging may exacerbate the illicit tobacco market, as it could be easier for counterfeit producers to replicate the plain packages than current tobacco packaging. ...

Well not having to have 100's of different brands to reproduce, and having them replaced by different lettering in the same font, may go some way to reducing the outlay required for (new) forgers to setup may go some way to exaserbate this problem...

... A way to counteract this potential problem would be to require other sophisticated markings on the plain packages that would make the packages more difficult to reproduce. ...

Since they don't suggest in detail what this might entail, I suggest that anything they can come up with will be not be difficult for the forgers of current packs. If it can be printed on the packs by the manufacturers then the forgers can do the same. If it's, for example, an external label produced by the likes of De La Rue, then all the forgers have to do is find someway to steal some from somewhere.

... In addition, the colour picture warnings, which must appear on all tobacco products manufactured from October 2008, would remain complicated to reproduce.

They're doing it now. They can't be that complicated.

3.80 If plain packaging was to be introduced, it could be more difficult for retailers to conduct inventory checks, and customer service could be made more difficult at point of sale. However, brands could be stacked in alphabetical order, for example, to facilitate quick identification, [...]

Well, since everything will soon be hidden behind shutters in the UK, and they're not working with branded packs, it's certainly not going to be easier with plain ones.

3.81 The introduction of plain packaging for tobacco products may set a precedent for the plain packaging of other consumer products that may be damaging to health, such as fast food or alcohol. Nonetheless, as tobacco is a uniquely dangerous and extremely widely available consumer product, it has for some time merited different regulatory and legislative treatment from other consumer products.

But the unstated problem here, is that the 'ban tobacco' model is being used elsewhere, and mission creep is already occuring in the areas of fast food and alcohol.