The desire to protect its trade routes with Asia, and particularly with India, was one of the most important reasons for Britain's involvement in the Scramble for Africa.
This can best be seen in British policy in what is now South Africa and in Egypt and Sudan. South Africa was strategically important in the time before the building of the Suez Canal. A hostile power on the tip of the continent could have done much to block English shipping between England and India. Of course, this was before what is commonly known as the Scramble for Africa.
During the time of the Scramble, the Suez Canal was in operation, meaning that Egypt and Sudan were much more important than South Africa. This led to the British being very aggressive in defending this area. This aggressive stance was seen in Gordon's attempts to hold Khartoum, Kitchener's expedition to retake the city, and the subsequent near conflict with France at Fashoda.
Of course, much of England's involvement in Africa was nowhere near Egypt and Sudan. Therefore, protection of trade routes was not England's only purpose in the Scramble for Africa. However, it was one of the most important reasons for being involved in Africa.