Coming in a bit late, but there is some important context that helps explain why the UK has done this.
First, the UK is involved in a complex, dynamic, set of international negotiations, both in the EU and in the UNFCCC on climate change. The logic is that because climate change is a global issue (all molecules are created equal), the end goal is to have the issue addressed by all the major emitters and potential emitters – countries to which emitting activity might relocate.
This is definitely not a one-shot negotiation, nor is it a zero sum game.
So it is all about building trust and trustworthiness on all sides. It is also about maximising the gains from trade that can then be shared (the hackneyed “win-win situation”).
This means that the high-income countries (and that includes us as well) have to demonstrate a willingness to incur costs as a way of building trust on the part of the low-income but high actual or potential emitting countries, who will also have to incur costs if a truly effective international solution is to be found.
(BTW, this is why the NZ Institute’s idea of NZ being a “fast follower” got up the nose of some politicians: it was exactly the wrong signal to send to the international negotiations.)
On the EU side, the EU has made an in-principle decision to bring aviation into their emissions trading scheme. So again, the UK is showing “leadership” (building trust) as a way of getting the rest of the EU into the mood to address the issue.
While difficult to quantify, I can see a lot in this line of argument. The rich countries are going to have to put on their hair shirts for a while. The trick is to only have to do so for long enough to make the international negotiations work.
The other reasons are far less convincing and smack of pretty bad government failure to me. They include:
a) The UK has just passed a Climate Change Bill that imposes binding, unilateral, emissions reduction targets on the UK (again, partly as a major signalling point to the international negotiations). During the parliamentary processes, the Labour back-bench wanted to include international aviation and shipping, even though they are, as of now, outside the UNFCCC’s scope. Partly about level playing fields (if domestic activity is going to be subject to restrictions, need to try and the international stuff in), partly about influencing negotiations and partly about addressing domestic political concerns (long-haul international travellers, especially of the high-income type, are getting some stick in the UK for being environmentally irresponsible).
So given that international travel is no included within the total UK emissions cap, a measure is needed to apply a price signal.
Now, as others have pointed out, there are other ways to do this, but since the political goal was to show that the parliament was listening to community concerns about rich pricks (to coin a term).
b) there is a strong push in the UK to apply instruments at the individual level, as a way of building “community” (this is the point were we start singing Vera Lynn songs and tell stories about the good old days of the Blitz). One particularly silly idea was that each person would get an individual “carbon credit card” that would include their portion of the UK’s Kyoto allowance and who have to use it whenever they purchased anything with a carbon footprint. If you used up your allowance, you would have to go and purchase a “top up”. The compliance costs were horrendous.
So putting the price of carbon on in a very transparent, but high cost and potentially ineffective manner is part of this trend.