how about you look at it in this way? If US and UK are not actively involved in foreign affairs like they are today, then some other country or a group of nations would take over the US and UK.
It cannot happen if you think about it, but my point is it is not the question of why -- it is more of who makes the first move - there is always the first mover advantage that you can stay on top of that.
If hypothetically, UK starts to be less concerned about international affairs and places more emphasis on domestic issues and resolving domestic challenges, someday this country would become perfect -- but more like a perfect recipe so that someone can take over. It is hard for anyone to agree with me on this one, but thats just the way it is.