Not mini eggs in particular, but I think supermarkets in general are buying less Easter stock so that they aren’t left with excess.
My DD worked in Tesco for a while - 3 yrs ago at start of lockdown she came home from her first post Easter shift (at a Tesco extra) with an armful of reduced E Eggs. Last year they had sold out before Easter weekend.
A medium sized Tesco near us had very few E eggs left last week, and the local Asda stock has shrunk down - replaced by Father’s Day sets!
Hope you find some mini eggs Op!