There’s an aspect of so what to the question: what would actually change if the Brits vote to go their own way in a few weeks? One set of answers involves British influence on the continent or within the EU.
[I]f the British do vote for Brexit and then have to trade with the EU, the UK would in fact find itself isolated with very little influence over regulatory standards in the single market.
Great Britain already has very little influence over those regulatory standards: look, for instance, at the hue and cry from the rest of the EU over how the British comport themselves, and regulate others, in London’s financial markets. Also, from outside, the British would be utterly free to set their own market regulatory standards without interference or “help” from their Continental betters.
It’s the same with welfare programs and immigration generally. Although the British do fare quite a bit better here—as part of British Prime Minister David Cameron’s pre-British exit (Prexit?) negotiations with the EU, he won concessions (not enough, some would say) regarding the amount of British welfare payments would be paid to migrant (not immigrant) workers and their stay-at-home (country) families. But the French already are up in arms over English impertinence at not letting immigrant/refugees to flow across the channel quickly enough to suit France.
Neither of those problems would be any worse with a British exit, and the British would be in absolute control over them were they to go their own way.
The upshot looks as though there is, indeed, a what to the so what—and it favors a British exit from the EU. Which, in addition to giving Great Britain a great deal more flexibility and complete national sovereignty, would restore the British to their historical role of power broker among hegemon wannabes on the continent.
That last isn’t all bad for the British or for the continent.