The power of a country is down to it’s military, economy and diplomatic influence. The United States for example is powerful because of it’s strength in all three. A country like the UK has economic power and military power, but in the long-term it’s relative position in these areas is likely to decrease due to it’s size.
The power of a country then influences how sovereign it is. Some countries can have large influences over other countries through economic and military threat or through a third party via its diplomatic relations with other countries. The more influence these countries have the less sovereignty a country may have even if from first look it is ‘self-ruling’.
UK
At one point it had a large global influence (superpower status), and had much power over what direction it headed. As this status faded as a consequence of the deterioration of the empire, its own ability to exercise it’s will on a global and local scale also decreases. It is now more beholden to the choices of foreign powers, whether this be via tariffs, making policy decisions to gain favourable trade, or many other ways in which it sacrifices its decision making for ‘progress’.
In cases where this hard economic power allows stronger decisions making, it may make sense to form military or economic pacts with other like-minded nations, and use this combined power to collectively bargain and increase sovereignty. While to individual nations these may seem like a sacrifice of sovereignty, if properly implemented (with good devolution), it could actually provide these nations with more power because of the inefficiency of fragmentation in policy that requires large capital or bargaining.