A milk float is the UK term for a milk truck -- a truck used to deliver milk to individual customer's homes. They tend to be small, boxy trucks with open-sided backs for easy access to the milk bottles, looking a bit like ice-cream trucks or tramcars. These days they are often electric vehicles. This allows them to make their morning deliveries very quietly. This also means that they tend to be rather slow, but this is not a problem when making delivery rounds.
These days milk floats are becoming less common, but are still running in many cities. Glasgow has the largest fleet of milk floats, and most large UK cities have at least a small fleet. These days, all milk floats are manufactured by Bluebird Automotive, although older models by other manufacturers may still be on the road.
The term milk float originated in the distant days of yore, when they were but horse-drawn carriages. Many of these carriages had the same boxy form as the modern milk floats, but depending on local needs almost any carriage or cab could be used. The term most likely comes from the low-body, crank-axle carts or 'floats' used for carrying particularly heavy loads in the late 1800s. However, an alternative possibility is that it may refer to frames or 'floats' added the back or sides of carts to extend carrying capacity, a term used as far back as 1686.