The point is that the plastic bag tax works because people pay it explicitly and separately. If you use a bag you pay a visible fee. But a coffee cup charge will inevitably be absorbed into the price of a coffee because it's not really a separate item. You can't just have the coffee and not the cup. People won't notice it the same way.
Admittedly, there is a kind of way to have coffee without a cup, and that's to take your own cup in. Starbucks, for example, have a 25p discount on takeaway coffee if you do this. So they are effectively imposing a 25p disposable coffee cup tax - five times as much as that proposed by the Lib Dems. And certainly some people do take their own, but it's an inconvenient thing to do, compared with a carrier bag you can stuff in your pocket. Based on observation on visits to the chain, it's a tiny proportion of people who do - probably significantly less than 8.5% rather than 85%.
For this kind of action to deliver you have to do two things - make the tax visible and easy to avoid by doing the right thing, then to monitor alternatives to ensure you're not just shifting the problem elsewhere. The carrier bag tax has done the first of these, which is great. I haven't seen any reporting on the shifting of the problem, however, which is a little worrying. I know that when Ireland introduced a similar tax there was a surge in production of other types of plastic bag to cover situations where people had been repurposing carrier bags, so the actual reduction in plastic film going into the environment was significantly less than the apparent reduction by merely looking at carrier bag use. I've not seen any figures for this in the UK yet.
Environmental measures should not just be for appearance's sake. They need to deliver a benefit and to have a measurable impact. And I find it hard to believe that the 5p coffee cup tax would be anything more than greenwash.
This has been a green heretic production.