If we leave, we are on our own.
Cites, trade restrictions, import/export duties to/from Europe. The only way around that is a trade deal with the EU, whereby we have to sign up to pretty much whatever they want. So we throw away what influence we have, gain nothing in return and have to go back cap in hand and get a worse deal.
Thats the reality, we would be a relatively small inconsequential country dealing with the likes of USA / China / SE asia and the EU and hoping they are going to be nice to us...
Sure if you want to be a minnow in sea of sharks, lets leave, we'll see what happens. If you think it isnt shark eat shark in this world look at the BP experience in the USA, or some of the trade deals the US has tried to impose on the EU. If it wasnt for the EU the US would have screwed us over a dozen times over.