I don't like Seymour as much as the next guy but what is the problem here? Undertaking isn't breaking any rules as far as I know.

Edit: oh is it that he's passing a car already doing 110?

They are both in the wrong here.

Guy being passed should be in the left lane. He is filming with his phone while driving and is speeding (technically).

Seymour is speeding faster. Is undertaking, while undertaking is not illegal, it is a driving convention that people expect because that's how people are taught. People generally are doing checks in their middle and right mirrors, anyone on the left is less likely to be seen.

They should give the driver who filmed it a ticket for using their phone while driving.