The UK government's stated opposition to the FTT was hardly unexpected. With the City of London acting as the financial heart of the EU, and a sacred (cash) cow for the UK government in terms of tax receipts, the UK was always going to be resistant to the idea. Barroso, in his State of the Union speech, seemed to recognise this and generally supported a two-speed (or multi-speed) EU.
When the national interest is invoked as a reason for a policy position, it shuts down debate. However, while national interest is part of it, since the UK government supports the idea of FTT in principle, provided it is applied globally, there has been a bit more debate on the idea. I have to admit that I don't fully understand the mechanics of how the tax would puch financial businesses outside the EU and outside the UK: my understanding is that the proposed tax would be applied to transactions where one side of the transaction was in the EU - so even if the financial businesses and banks moved outside the EU, they would have to pay the tax if they wanted to do business in the EU. It would only make sense to move if the business did most or all of its business outside the EU. However there are good points on the fact that a large proportion of the tax would be collected from the City of London, and this would be unfair if the Eurozone mainly benefited. If the income was used to build a safety net for the banking and financial system across the EU (to reduce the burden on taxpayers in the real economy), than that would probably be fairer.
While the Labour party in the UK will probably support the government's resistance, it would be interesting if they decided to support an EU FTT in some form - after all, their leader Ed Miliband has referred to businesses which were "bad" for the economy: would the FTT not make sense in rebalancing these ethical issues by making the financial industry pay a bit more tax to insure against the danger of being (ultimately) underwritten by the taxpayer? The BBC's Robert Peston has an interesting take on the FTT here.
In any case Member States have a veto on the matter, so the UK can block it. But the implications for the EU of a Eurozone FTT haven't received much attention. We already have a multi-speed EU, with some countries in or out of the Euro, the Schengen Zone, the EEA but not EU Members, etc, but these have been in different areas of integration. If you start to adopt different speeds to the internal market in a way that affects the four freedoms, then it could cause some political headaches. It would raise the EU's West Lothian Question. Why should MEPs from the slower countries have votes in areas where their countries aren't affected? Already the British Commissioner couldn't (politically) be the Commissioner for monetary policy since the UK is not a Eurozone member. The more the Eurozone countries pull ahead, the less influence those outside Euroland will have.