Taken from Manufacture and Storage of Explosives Regulations (MSER) 2005.
a) Storage container size
The HSE recommends that all shooters powders should only be stored in containers holding less than 550 grams. However for the time being, until stocks are used up and replaced, some powders may only be available in 1kg containers. It is permitted to continue to store powder in these 1kg containers until manufacturers reduce the container size, the conditions in relation to the storage of black powder continue to apply, irrespective of container size i.e. a wooden partitioned box.
I believe that this information may be slightly out of date as I know there was a fairly recent update, but basically it is my understanding that retailers are not permitted to sell reloading powders in the 8Lb tubs that are common in the U.S. I have known of dealers buying powder in bulk and then decanting it into smaller containers for resale but have never seen the large containers for sale in this country.