Well, the Celts got distracted by the influx of Germanic tribes and as such had more immediate things to worry about and hate than the Romans, but I figure if the Franks, Saxons, Vikings, et. al. decided to stay home, the modern Britons would still grumble about the Romans occasionally.

I mean, the Germanic invasions started over a millennium ago and dislike of that's still on a low boil, so I figure two millennia isn't out of the question.

On the other hand, the Romans did go home. The Saxons, not so much.