The sovereignty issue is just one of the many things that are unknown at the moment, and won't be clarified if and until after the die is cast in favour of an exit. What the trade agreement will actually be, what the border control situation will actually be, whether or not we will still be a signatory to Schengen, what the impact to our financial and service industries will be, what happens to farming and fishins industries, many of whom are set up based on EU law and regulation, how an exit would affect our relationships within NATO, what difference this will mean to our armed forces, who are actually now part of an EU force even though they remain apart from that structure in all but the operational deployment sense, what then happens to the issue of Scottish Independence, how freedom of work and freedom of residency are affected, what happens to the ex-pats already living in Europe with regard to pensions, access to healthcare, and their continued ability to live and work abroad, what happens to the EU nationals living here (same subset of questions) etc. That's a very, very long list of unknowns.
Sovereignty is a very handy peg to hang an argument on, because nobody really knows what it means. People say that it's about being in charge of our own destiny. Many countries don't have a sovereign but are independent. It's about how much of our lives are influenced by Europe as much as anything, and that has always been, and will always be, a moveable feast based on the collective will of the member states - us included. And if we vote no, I hope that everyone understands that when people talk of a 'leap into the unknown' - remember the list of 'what if' points above...