Since the election the backing for Juncker as the European Commission President has stiffened, from Angela Merkel to even European Left candidate Tsipras saying that Juncker deserves to get the job if he can get a parliamentary backing, because that's the democratic, parliamentary process. Set against this, Cameron's campaign against a Juncker presidency looks more and more misguided by the day. While the Netherlands, Sweden and Hungary are against Juncker, by leading such a prominent campaign against him, Cameron has probably made it more likely that Juncker will get the presidency.
Cameron tends to have a very short-term, tactical approach to politics, particularly when it comes to the EU. The big speech that was supposed to outflank UKIP clearly didn't - in fact, the more the Conservative party echoes UKIP, the better UKIP seems to do. Promising a renegotiation and an in/out referendum on the basis of the result hasn't done much to quiet his party on the matter either (with people now wondering if the referendum date of 2017 will be brought forward to 2016). When wielding the veto in December 2011, Cameron was able to strike the pose of a decisive leader with good Euroskeptic credentials, but the ability of the other countries to go on without the UK ended up showing how devalued a veto can be. (If the UK had taken part in the negotiations while holding on to the veto, it would have been able to shape the agreement and it's potential veto would have carried more weight towards the end of the process). This short-termist thinking is often traced back to Cameron promising to take the Conservatives out of the EPP as part of his platform for the party leadership.
Opposing Juncker's candidacy was always going to be risky. While the campaign might not have been as high profile as some would have wanted, it's difficult to name alternatives from outside the Europarty candidates - Lagarde has ruled herself out, and many of the others are serving Prime Ministers and Presidents. Pascal Lamy, director-general of WTO and formerly the chef de cabinet of Jacques Delors, is hardly a fresher name than Juncker when it comes to EU politics. More importantly, the European Parliament is set on getting one of its candidates in the job, and they can veto any nomination by the European Council. It's difficult to see any of these speculative alternatives giving up their jobs (or present themselves as willing to give up their premierships/presidencies to their national electorates), to place themselves in the middle of a power struggle between the Parliament and the Council.
And while the UK has some allies on its side, it's going to be very difficult to form a blocking minority in the European Council. Cameron's view on EU politics seems strikingly simplistic - focused on winning over Merkel and co-opting Germany's political weight in Europe. It's hardly a secret that Merkel is lukewarm on a Juncker presidency, but the German media rallied behind Juncker when it was suggested that the UK may be threatening leaving the EU. That the CDU's coalition partners in Berlin, the Social Democrats, were so closely wedded to the presidential campaign meant that the pressure on Merkel to publicly back Juncker was strong within the government too. Rather than working to quietly sideline Juncker behind the scenes, Cameron has made it much harder to get rid of him by forcing public declarations of support or opposition.
All this raises the question: if it's this difficult to block Juncker, how much influence would the UK have in shaping the alternative? A nomination still requires a majority. Even if Italy joins the UK in blocking Juncker (a big if, in my opinion, as Italy will soon have the presidency of the Council for the second half of 2014 and will probably want good relations with the Parliament if it wants to push legislation through), will the blocking minority form a coherent enough bloc vote to be able to shift the rest of the Member States (and for the UK to have a decisive role in that)?
Cameron would probably have done better by quickly getting the European Council to adopt priorities that are closer to his position using the election results as political support. The European Council still sets the overall policy direction of the Union (and Merkel has tried to steer the debate in this direction as a way of finding consensus). But now a Juncker presidency will have been badly burned by the right-wing, British-led, opposition to him, and he will be aware that his political base in the Parliament rests on a coalition with left-wing parties who were needed in order to overcome ECR opposition and any EPP rebellions. The institutional balance and Juncker's own affinity for fellow national leaders will mean that his presidency will focus more on consensus rather than confrontation, but it will be far from a natural ally of the current UK government.