European Union Bill

Wisely, Britain already has a number of opt-outs from the European Union. I am thinking specifically of the single currency; it was to the great credit of our former leader that he kept us out of the euro. Would not a test arise, however, if Britain decided to opt out of something that we currently opt into? For example, if we chose to withdraw from the common fisheries policy and to place our own historic fishing grounds under democratic British control, would not that represent a test of our sovereignty?