The Egyptians domesticated the ostrich and it has been familiar to Mediterranean civilization since. England was part of the Roman world just a few hundred years before the period in question. At that time at least, there were lions in London's zoos and gladiatorial games. I don't know of records to this effect, but there might even have been been ostriches - a favourite animal to import for the games elsewhere in the empire.

It's much less likely there were live ostriches in Anglo-Saxon times but the rich have always enjoyed mangeries and exotic animals. I wouldn't be surprised. And there would be traveller accounts and stories, increasingly mythical to be fair, as large population movements from places where people had actually seen them declined in the post-Roman era.