Back in the days of peak "booze cruise"[1] the ferry companies used to issue every passenger with a leaflet describing exactly what the duty free limits were. You'd go over, max it out and come back.

If border control were to select on this basis they'd spend all day pulling over bearded men in anoraks driving volvo estates, searching their cars and discovering that they have exactly the "right" amount.

[1]: https://en.wikipedia.org/wiki/Booze_cruise