Do not confuse banking with capitalism
Clearly the bankers f***ed up. Which is why the markets punished them. They were bailed out by the government, not by the markets because the markets said, "you've failed, you are insolvent, so we won't lend to you anymore, your stock price should be lower!". And when stock prices fall, we get hurt (if you have any savings that is, I don't!)
All of these failed bankers would have been wiped out if the government had not bailed out the banking sector. And they are still doing so via the Bank of England's policy quantitative easing, allowing banks to borrow at 0.50% and lend to ordinary folk and small business at 10%+, pocketing a risk free return at taxpayers expense. Not an ideal situation!
However, a financial transaction tax would harm savers, pensioners and small businesses such as farmers and airlines that require liquid and competitive commodity and financial markets to hedge against sudden price movements in wheat, cotton or oil, etc or indeed the future financing of their business. We need more competition in finance, not an industry concentrated in fewer and fewer hands (=large banks) with close ties to government. Just think of the rip off rates you pay when you exchange currency to go on holiday, or even the charges levied on you by your local bank. A financial transaction tax would reduce competition, so the banks would probably benefit overall, if you look at the big picture.
Even if you applied a financial transaction tax within all of Europe, other nations such as Singapore, Hong Kong and China are looking to grow and would not follow, they would laugh at us. And why should they follow, why should we be allowed to decided another sovereign nation's tax policy?! We don't have an empire any more folks!! Financial markets are electronic and global, so the result for the UK economy would mirror the Swedish experience, financial business would move elsewhere...
So really when you support a financial transaction tax you are basically saying you want to destroy the UK financial services industry and that falling tax revenues is a price worth paying. The problem is that the tax revenues from all industries are needed to pay for our welfare state, and if you reduce the tax revenues that are received by the UK government by killing a particular industry, like in Sweden, you then become more dependent on international financial markets for funding the government's spending on schools, hospitals, education...
Do we really want to be in a position where the UK is begging the financial markets of other economies, or the IMF even, for a loan on their terms? NO!
We should spend more time trying to develop new industries via preferential tax rates or subsidies, whether in IT/internet, bullet trains, electric cars, new forms of renewable energy, maybe even smaller regional banks...not wasting our time trying to kill industries. People will move from banking into other industries are exciting and growing, the problem is we don't encourage new industries to grow in the UK. We have all become socialists