Either you are...or you aren't.
Yes, but it's important to understand the context here.
Even if the UK leaves the EU completely, it still wouldn't achieve a complete reversal of the changes that have limited its sovereignty over the last few decades.
As one example of this, in addition to being a European Union member, the UK is also part of a completely separate and older organisation called the Council of Europe. It will remain part of the Council of Europe even if it leaves the EU.
And that means it is still subject to the rulings of the European Court of Human Rights (which is a CoE court, not an EU Court).
The CoE is less of a restriction on the UK than the EU is, and the ECHR is less interventionist than the organs of the EU, but it's still some restriction on total sovereignty.
Some of us hope that after leaving the EU, there will be further consideration of withdrawal from the jurisdicition of the ECHR, or even total withdrawal from the Council of Europe, but one problem at a time.
Just getting out of the EU will solve 80-90% of the problem. It's not everything but it's the right thing to do first.
The extent to which ECHR and CoE membership are sovereignty issues is, to put it mildly, debatable. Turkey and Russia are members. It’s hardly conceivable that either of them would be so if they believed their sovereignty to be affected!