A quick check tells me that 4,462,270 jobs in the UK that are maintained by the UK trade with the EU, amounting to 15.74% of total UK jobs.
Now I'll be straight with you. I've no idea what would happen if the UK pulled out. i doubt all of this trade would be lost. But I don't live in the UK, and i do work for a multinational company, and I can tell you the trade barriers that exist between the EU and, say, the USA make it nigh on impossible for us to export there. Trade to the middle east is difficult due to the mad levels of paperwork, and to countries like Norway is hampered by incredible customs difficulties.
Before The UK joined the EEC, only 13% of your exports were to European countries. it's now 55%. Your population is simply too large to close the borders to trade.