Pat59 - diagnosed with Essential Thrombocythaemia in 2018. My hospital doesn't consider ET a cancer, even though WHO does. Why would this be?

Hi, I was first diagnosed with ET in 2018 and started Hydroxycarbamide. I attend a hospital in Glasgow although I am cared for under Haematology not Oncology. I've been told that the hospital does not consider ET as a Cancer. My question is, "Why would a centre of excellence in Cancer care say ET is not Cancer when the WHO says it is?"