There were five Milk Marketing Boards ...
1. England and Wales
2. The Scottish Milk Marketing Board
3. The Aberdeen and District Milk Marketing Board
4. The North of Scotland Milk Marketing Board
5. The Milk Marketing Board for Northern Ireland
The Boards were established in the Agricultural Marketing Acts of 1931 and 1933 (1955 in Northern Ireland). The DEFRA website states that in 1943 there were 100,000 producers of milk. The boards guaranteed the price of milk to all producers (about 12d per gallon in 1934 depending on quality).
When the Boards were abolished in 1994, there were around 28,000 producers in England and Wales. There are now around 13,778.