Milk is around 90% water, so it makes sense to stabilise it as butter or cheese or milk powder before trying to move it anywhere. A good example would be New Zealand, which has a network of butter/skim milk powder plants, where milk is centrifuged to extract the cream. This is made into butter, while the remaining skimmed milk goes to a drying tower and leaves as skim milk powder (SMP).
The UK has a tradition of marketing its milk as liquid milk, which is either pasteurised, sterilised or put through an Ultrahigh Heat Treatment (UHT) line. UHT cartons will have a life of around a year at ambient temperatures, sterilised milk should be protected from the light, but will last indefinitely in unopened bottles.
Using a centrifuge, milk for pasteurisation or UHT lines will be standardised at 4% or 2% or zero percent for full cream; semi-skim or skim respectively. Surplus cream is usually collected in a tank for sale on the industrial market. A milk packing plant can quite easily generate a tanker load during the course of a week, for most of the year, with the exception of a Christmas peak in UK sales of cream. Before Brexit, tanker loads would often travel as far as Germany and be a viable proposition.
Milk powder footnote
All milk for powder making must be centrifuged before it goes through the drying tower. Any remaining cream or protein would block the fine nozzles used to spray the milk into a rising column of hot air. The fat-free Skim Milk Powder is collected from the bottom of the tower.The fat content is restored if there is a need for Whole Milk Powder or a custom formulation for food manufacture or baby foods.