Shouldn't this be the United States and China, given that Britain doesn't have much of an empire now and isn't committed to spending vast money in infrastructure in Africa and Asia.
But I'll bite. We can have the powers on the UN security council--US, UK, France, China, and USSR/Russia, doing something like this. And there's a lot of cool canals and river improvements to be built. Sadly, the best in the US have run into funding problems. This is why the Trinity River canal was too little, too late, even though Dallas, TX could easily have been a good inland seaport for exporting oil and crops. There's also the demand issue--the Missouri River is in theory navigable to Great Falls, Montana, but there's not enough to demand on river transit to keep the river navigable for barges north of Sioux City.
Building new lakes and rivers could work too, and this would be good for development. France could have the chotts of Tunisia and Algeria flooded to build the Sahara Sea which would cool the local climate, improve agriculture, create salt lakes to mine important chemicals, and allow navigation by sea to Biskra in Algeria. In theory, you could power hydroelectric energy with the fall of the water (as it is below sea level in the inland parts) which would allow you to power desalination plants to turn some of the lakes into completely freshwater lakes to irrigate land for agriculture with. A similar thing goes with the Qattara Depression in Egypt, and several small flats in Libya. Australia has Lake Torrens, which unlike Lake Eyre, is far better to flood and would expand the Spencer Gulf further inland to enable easier access to the ocean for mines in South Australia.
The Niger River also appears a good target for improved navigation. As one of West Africa's major rivers, I think a good canal system could be built allowing year-round (or almost so) river transit to Niamey for river vessels, and maybe even as far as the Inner Niger Delta.