No country had a very significant impact in both of these phases of the British presence in Africa. There were countries that impacted the British in the Scramble for Africa, and there was at least one country that impacted it significantly in decolonization. However, no country had an impact in both phases.
In the Scramble for Africa, Britain was impacted by at least three other European countries. These were France, Germany, and Belgium. (Italy, Portugal, and Spain were more minor players in this competition.) All of these countries were pushing to gain empires in Africa and impelling Britain to gain a larger empire to compete with them.
The United States had the greatest impact on decolonization. It wanted Britain to decolonize largely to make the West look better in the Cold War. None of the European countries had any significant impact in pressuring Britain to decolonize.