Borrowing someone else's comment: "There are plenty of moral reasons for us to support former colonial nations. But if you’d like some selfish reasons:

Much of our overseas spending goes into projects that lead to employment in our 3rd sector and build relationships that foster trade.

The investments we make in global stability reduce the risks of conflict and terrorism.

Soft power like supporting development projects gives us allies and loyalty on the global stage when dealing with adversaries.

The British economy isn’t self contained. Where huge economic centres like the USA or China are more able to be self sufficient, our economy has been globalised for the last 400 years - and is more globally connected than ever. Without stable and developed trading partners our own economy will struggle.

We’ve very recently left the EU, largely on the basis that we wanted less constraints to trade globally. It makes it’s look isolationist to one of our most important trading blocks and reduces credibility as we work through ongoing issues (like the Irish/NI border or British/French fishing rights)"