One of the most powerful arguments for those who support the ‘leave’ option as well as for Team Cameron’s renegotiated terms is the need for Britain to reassert its parliamentary sovereignty and control over each laws. This also means not being subjected to European Union's drive for “an ever closer union” whose most important institutional aspect is the European Court of Justice’s interpretation of what these groups claim to be intrusive European laws that are in the process of changing the very DNA of the British legal system. Here we must distinguish between procedural sovereignty and substantive sovereignty. Procedural sovereignty means that 600-odd MPs in the Palace of Westminster have the legal authority to debate and adopt any laws they wish for the duration of their mandate. Substantive sovereignty means the ability to effectively shape and change outcomes for specific issues and challenges by means of government legislation. Whilst it is true that leaving the European Union and its legal system would give back to the Parliament of Westminster the procedural sovereignty enabling it to pass unimpeded any laws it wishes, its actual ability to have a real effect on a wide variety of issues that can no longer be effectively resolved solely by the United Kingdom will vanish.
Source: BBC1 HD, Andrew Marr Show, 10 January 2016
In effect regaining parliamentary sovereignty means having more and more control with less and less influence over fewer and fewer issues. Not only will we not regain a Diceyan parliamentary sovereignty that no longer is applicable to the 21st century, but by excluding ourselves from those institutions with real ability to affect outcomes that can only be solved at the European level we will in fact surrender our capacity to have a voice and a vote in determining our futures.