The germans were convinced that they get the colonies during a victory in Europe and they also meant british colony, otherwise i cant see why the wargoal "Mittelafrika" was considered as one of the most important war goal. also why would some african colonies be important for britain, they still have India.
IF the germans knew that it would be impossible to gain those territories i doubt they would even consider it, that would just be a waste of time.
you dont have to occupy London in order to bring the british down to the knees, nor do you need a naval victory, a britain which is exhausted from the war even more than the CP are, a britain which is alone and has money problems, would may agree with the german claims, why risk that they country ends like Russia, why not just accept and give them some of the rather unimportant colonies in Africa ? They still have South africa and the whole India, those two are most important.