Marv and Angelus have already addressed the left's major issue with regard to the EU - the neoliberal agenda - and I don't have much more to add to that. But, I think you're mistaken to assume that retention of national fiscal policy is a concern for social democrats. The social democrats in the EU are in fact strongly behind monetary union. Opposition from the further left is sometimes a matter of principle (what you might term "nationalistic" reasons, what others might see as localising democracy) and sometimes based on the simple fact that monetary union has and will continue to operate in the interests of the Euro-elite. The austerity measures have been brought in under absolutely massive pressure from the ECB, in order to protect the single currency. Now maybe they would have been brought in if there was no single currency, too, but at least then there would have been other options available and the opposition could realistically promote those other options.
On the issue of British insularity, it seems to me that you're taking a tautological view - they're opposed to the EU because they're insular and the fact that they're insular can be shown by their opposition to the EU. But, you know, the streets of London are as cosmopolitan and multilingual as anywhere else in the EU - despite their supposed insularity (and the fact they are not part of Schengen).