Cameron is far more worried about UKIP than he is the EU, and rightly so as far as his own political fortunes are concerned IMO.
The problem for Cameron is that he thinks he is demonstrating his willingness to pick fights with the EU, while what most UKIP voters actually see is his inability to win any of these fights. He's not going to attract any of them like that.
Which is precisely why sooner or later Cameron will have to be willing to let his bluff be called and let the Brusselcrats do their worst. Since there doesn't seem to be any automatic tat in place to respond to Cameron's tit, this actually seems to be a reasonably good place for him to make a stand. By the time Brussels comes to any decision on what to do, the next UK general election will likely have passed.
The problem for Cameron is that because of the way the British election system are set up, it mean nothing to the rest of EU that UKIP get votes from the Tories, that will jut mean that Labour are more likely to gain power, which wouldn't be a problem for anybody in EU. So all this depend on Merkel, Hollande or... well anybody with influence in EU doesn't give him a nice push, it's not like he has a lot of friends in EU or the other European capitals, so I think it's very optimistic to expect anybody to do him a favour.
If Cameron did nothing in response to this short notice tax bill, or if he caves to the Brusselcrats too soon, that is indeed what will happen. However, as I mentioned already, Cameron likely won't face any real consequences from Brussels until after the 2015 election is over, so it makes perfect sense for Cameron to grandstand until at least then. The danger as far as the EU is concerned is if Cameron both wins the election and has been forced to grandstand to such an extent as to make Britain leaving the EU a real possibility even if Cameron tries to dial back the rhetoric after the election.
I'm sort of reminded of the secession talk that took place over here prior to our Civil War. The North pooh-poohed the possibility as being against the South's economic interests, which it was, even had the South departed in peace, largely because previous Southern threats to leave had never resulted in anything. It's still a low probability event that Britain will both hold a 2015 referendum on staying in the EU and then go on to decide to leave, but continued pooh-poohing of Britain by the Brusselcrats will continue to make it possible and also serve to make leaving after some post-2015 UK election be decided upon at a rapid pace designed to forestall the possibility of compromise.