Osborne is against the European FTT because it would “hit” transactions between UK and other European organisations. We would have to collect it, but then pass it over to the EU.

More details here: http://www.bbc.co.uk/news/business-15552412

I don’t see much of a downside. These sorts of taxes are always less than one percent, so the money gathered, though the overall sum is large, is minuscule compared to the value of the transactions that have been taxed. If you think that less than 1 percent will raise, say, 30 billion, that means 30 billion is less than 1 percent of the value of the transaction. That’s a seriously fucked up amount of money.