Thanks. At least this article (partially) answers "Why am I supposed to feel responsible for the actions of people that were geographically and ethnically distinct from my ancestors, simply because they happened to share the same skin colour and kingdom?", but one first need to (as said in the article) "fully grasp the full benefit and privileges they have received" as a group, not a matter of rich vs poor, otherwise the point of the article is probably moot, especially if you've never heard of red lining or gerrymandering.
The article was referring to the USA situation. The difference between their situation and the Romans is that some of the people who played a part to the present situation (policies, laws, housing, blocking access to education...) are very much alive.
> Not many people in industrial Wales seemed to share in the benefits of colonialism.
I guess Welsh economy also profited (to a lesser extant) from slavery [1][2]. But if Normans purposefully enact systems that benefits them at the expanse of Welsh, solely (or even in part) because of their welshness, then of course you are legitimate to fight against it (if I understood you correctly).