I'm sure they got buried in Northumbria, and Bernicia, Deira, and Elmet, and Strathclyde (the last two were Celtic not AS). But there aren't many burial sites known in all Britain anyway, there's been a fair amount of erosion round the coast which could have destroyed some burials (especially Deira), and other burials could be masked by later structures, like cathedrals. I think Youk Minster and Ber=verley would be good places to look if they'd let us pull them down.