I don't think Cameron wanted to leave the EU when he argued vigoriously against FTT and onerous EU regulations on financial services, or weakening of UK's own bank and market regulations. In other words, Cameron wants a vibrant and properous City of London, maintaining its role as the banker and banking capital of Europe - that's where the big money is, not by being isolationist.
So this means stay pro-Europe and respect his coalition partner Nick Clegg, but continue to block FTT. In Germany, it's similar with Merkel opposing Cameron on wanting FTT, but her coalition partner in the FDP insisting on no FTT to strike a deal with Cameron.
Maybe, this side deal was made. No FTT or onerous bank regulations for Cameron to sign the fiscal compact and show unison on Monday. Then maybe the EU can pick up strength in their bailout of Greece. Remember, without a signed fiscal compact - and all 27 is better - Germany won't loosen up the money for the bailouts.
Cameron is not giving up much here, as the fiscal compact is on fragile legal ground anyway. Cameron also wants Germany to loosen up the money and for bailouts to pick up steam. A meltdown is not good for the City.
Cameron is playing this right. Der Spiegel says the fiscal compact is probably not legally binding anyway.
Legal Loopholes: Critics Question Merkel's Fiscal Pact Proposal - SPIEGEL ONLINE - News - International
http://www.spiegel.de/international/europe/0,1518,811791,00.html. PS, just saw Rantany's link with this same article, thanks.
Bottom line, all we care about is no FTT or stamp-duty-plus in Europe and we want the bailouts to fix Europe rather than have contagion. We don't need Cameron to be a euro-skeptic.