I don't think it's unrealistic to hope a law will come in place one day to make it compulsory for an overview of the Empire /Commonwealth to be taught in schools - in the same way e.g. Germany teaches about the Holocaust (To clarify I'm not comparing the two directly, but giving an example where a nation has not shed away from its past).
It doesn't need a law (in England). It just needs to be included in the National curriculum. That doesn't apply to Academies, of course, but I understand that most Academies follow it.
The Holocaust is currently the only compulsory topic at key stage 3 in the National Curriculum.
Key stage 3 pupils have to cover particular periods. The development of the British Empire and the trans-Atlantic slave trade are both provided as examples of topics that might be taught in the 1745-1901 period, along with Irish home rule and various other topics.
Indian independence and the end of empire is given as an example of a topic that might be taught in the 1901- present day period.
Those topics are not compulsory, but I imagine there are many pupils that study them.
As others have said, the difficulty is that there is an awful lot of history! If you make more topics compulsory, there would be less scope for studying other important ones.
I think the new government has said that it will review the National Curriculum, so it will be interesting to see what changes it makes. Start lobbying your MP!