> The order defines Class M1 vehicles as “those falling within class M1(a) and class M1(b) as specified in Schedule 1 of the Vehicle Classes Regulations, which refers to another bit of UK legislation.

> Oxford’s congestion charge is almost certainly enforced by cameras that scan your number plate. An ox-drawn cart doesn’t have a number plate, so it won’t be charged. Other vehicles like a Renault Twizy or Reliant Robin do have number plates, so they’ll be charged even though they’re technically exempt.

So there's not much to it: Plates are scanned -> the number is checked in the vehicle registration database -> not class M1(a) or class M1(b) -> no charge.

This goes for ox carts, Twizys and Reliant Robins.

The person didn’t actually ride an ox cart anywhere.

They looked at the law, saw “motor vehicle” and said that an ox cart doesn’t have a motor so should be fine.