It was the British Empire, not the English Empire. The union was formed partially because Scotland was flat broke from failing to colonise what's now Panama and needed access to the English economy while England wanted to prevent a perennial French threat on its northern border.

Also Americans are in no position to speak about imperial decline at the moment! Iran has a nasty chance of becoming their Suez unless competent leadership is restored pretty sharpish in my view.

The empire was British, not English, so those other countries that the English "share" the island with were a part of that.