That treats the European Communities Act as typical of other types of statute, doesn't it? Your example of the territorial waters and the radio licensing is simply an example of a piece of legislation which created an ambulatory -- had an ambulatory scope by definition. The double taxation treaties also appear to be on one view in precisely the same category; they are simply treaties which by definition only implement international agreements to the extent that such international agreements are there, so that they are variable.
The argument against you on the European Communities Act is that it is a very special measure, which not merely is silent on the question of withdrawal but by its silence actually excludes withdrawal. It assumes, it proceeds on the basis that a new legal order is now part of the United Kingdom legal order.