In England, the law was introduced in response to ring rot outbreaks in Poland. In 2012, the Food and Environment Research Agency reinforced the guideline, by demanding a ring rot test certificate accompany the spuds.
It is illegal to bring potatoes from Poland to the UK. The Polish Potatoes Order 2004 makes it illegal for someone to "import into England potatoes which he knows to be or has reasonable cause to suspect to be Polish potatoes.