Connecting this to Brexit is really distasteful and misleading.
The Roman Empire no longer had the will or material to support Britain and left them to their own devices against they’re will. They had no political or military systems in place to replace the Roman support.
None of this has any correlation with today.
Silly comparison. Brexit is nothing like when the Romans abandoned Britain. If you want to look anywhere for an example of the "Dark Ages" post Roman Britain is the best example of it. I don't think that will be the case in the post Brexit UK.
I'm an outsider, but I read that there was a feeling of a democracy deficit happening.
The direction of history is not always towards progress. One of the reasons why I think people felt like it was safe to make a protest vote for Brexit is because in general things have gone very well for the Western world for the last fifty years. It doesn't feel that risky to throw a wrench (spanner?) into the works.