Could you explain how these buying groups operate, when and why they were established, and we can proceed from there?

Most of the buying groups were established 35 to 40 years ago by independent individuals, not large chains or suppliers. They were created to compete against major chain stores, such as Howdens in your case, and against large stores in Germany and Holland that sell not only kitchens but also furniture, bedrooms, chairs, tables, and so on.