As my uncle always said if I tried to delve into the historical causes of modern problems, "...if you go back far enough, people used to eat each other." What will Blair apologise for next on our behalf? Cannibalism?
The reason why the Celts are still around to bother the Anglo-Saxons who came after them is that the Anglo-Saxons, unlike the Celts, did not butcher and eat bits of their defeated enemies. Maybe we should have done? On the downside, we'd have to apologise for it now. On the upside, it wouldn't be Tony Blair of that ilk doing the apologising or Gordon Broon waiting in the wings to take up the mantle of hypocrisy.
Only Denmark abolished slavery before Britain. The British Navy suppressed it wherever it could reach. We have nothing to apologise for in this connection. Perhaps the nations who still practise it (buying small African boys as camel-jockeys, for example) might like to get around to abolition now? They can wait 200 years to apologise apparently.