As a strong British patriot I would not want to see the United Kingdom to become completely submerged under the US sphere of influence. Although possibly the US's most important ally, you cannot say the United Kingdom was a satellite state of the USA. The UK was more than capable of operating by itself (for example the Fawkland's War, which had no superpower, UN or international assistance), and becoming a nuclear power in 1952 resulted in it having a much more important say in international affairs. Additionally, at the beggining of the cold war the British Empire was still mighty, which would have meant the UK would still have had a huge influence on not just her colonies but neighbouring and foreign nations, including both superpowers, despite decolonisation later on. The UK also became a member of the United Nations Security Council, allowing it to have a VETO, which could overule any decision made - including one made by either of the two superpowers. All of this to me sounds like the UK had an extremely influencial position on the world stage, which should result in increased influence in the game.
This does not just include the United Kingdom however, France, as many people have mentioned, should have similar powers and influences, along with China. Therefore all members of the United Nations Security Council (UK, USSR, USA, France and China) should have the choice to have their own sphere of influence, or be part of another. We must not forget that the USA and USSR weren't the only two players in the Cold War.
The beauty of the previous Supreme Ruler titles let the player take full control of the nation, and steer it to whatever end the player wished - which leads to more exciting gameplay. Who knows, it could even result in previous allies becoming rival emerging superpowers, adding to the 'what if' factor of this potentially brilliant game!