Someone hasn’t considered that building the foundations of your society on the oppression and colonization of others means those foundations become untenable once the colonized are freed, something that will inevitably happen because the colonized will relentlessly pursue it, and Britain’s inability to do any of those things is a result of centuries of decolonization efforts (well that and losing in competition with the other big imperialist), causing the yields of its imperialist efforts to wain, meaning Britain must either double down on these imperialist endeavors or reform the basis of its society, and so decolonization efforts are of immediate and critical interest to both imperialists and those who desire social reforms, or as he puts it, making things better in Britain.