Financial services in toto 7.5 percent Don. But the part that Camoron wanted to protect was much much smaller than that. HUtton, writing in the Grauniad says "

There might have been a case for David Cameron to veto the use of the EU treaties for the eurozone bailout if Britain's national interests had really been threatenedk. But they were not. Much of British finance in whose name Cameron exercised his veto – routine banking, insurance and accounting – was wholly unaffected by any treaty change. The financial services industry in Britain constitutes 7.5% of GDP and employs a million people; the City represents perhaps a third of that and, in turn, that part threatened – if it was threatened at all – some fraction of that. This is a tiny economic interest. If the coalition is serious about rebalancing the British economy, it is preposterous to place a fragment of the City at the forefront of our national priorities."





Q - source please. AFAIK the UK's trade deficit with the EU is pro rata smaller than that with other places so a reduction in trade with the EU will have a negative effect. And if the UK leaves the EU (this does look a it of a 1-way street for that) subject to GATT rules the EU will be able to erect tariff barriers and "measures equivalent" against the UK.