Some left-wingers place themselves in the Leave camp on the specious ground that this would enhance "popular sovereignity" and "democracy in one country" Even if it was the case, what difference would a slightly enhanced political democracy in Britain make? The traditional leftwing/Social Democrat policy of "reformism in one country" has always failed, one reason being, precisely, the international nature of capitalism. Which would remain the case and in the end force any breakaway UK government to put profits before people to maintain or enhance British capitalism PLC's competitiveness on world markets, which has been the fate and experience of all Labour governments here.
Britain withdrawing from the EU may give an elected government here more formal control over what it decides but not any more real control as it's not governments that control the way capitalism works but the other way wrong as governments are forced to give priority to profits and conditions for profit-making. As a capitalist trading nation on its own outside a bigger block Leaving might even give the government less control. The argument is also based on the illusion of "reformism in one country" which has tried and failed many times. You can't fight capitalism exclusively in one country. For the same reason, you can’t have "socialism in one country" despite what the Stalinists say. This strand of leftism that has been drawn to state capitalist versions of what it misleadingly calls socialism has been a constant source of confusion from the word go.