there is only 1 milk supplier to food chain - IoM creameries (I think that is name but check) - also makes cheese (some quite good ) cream (again considerably more expensive than UK supermarket prices but double cream is much thicker (more fat) than UK) cottage cream etc.
Don't forget Island has only 90,000 inhabitants - two competing chains would fall prey to first supermarket chain that tried it - Manx Ices (though aided by internal problems) was effectively destroyed by the supermarkets insistance on importing frozen hydrogenated oils and being allowed to label them ice-cream , that company was I believe a major consumer of Manx cream and after its demise (its plant is now the major junk food outlet on the Island) the IoM Milk Marketing moved into cheese. Milk was imported in past but only as 'sterilized' (long life) - many supermarkets offer lower priced 'long life' cream