You can buy in the seed in cwt or 500kg bags with a variety of different dressings on it, I believe from memory C1 or first generation seed is what you buy in for seed production and C2 which is the seed off the previous harvests C1 is what would be bought in for malt/milling/feed grain production.
The two farms I help out on both grow their own seed for the next year, I believe that hey do have to pay royalties to the breeders for this though. All dressing and treatment was carried out by Hutchinsons with a mobile seed dressing rig on a lorry on one of the farms, the other does it himself with a 'Heath Robinson' type setup in the barn consisting of a Boby dresser with a sprayer pump cobbled onto it :D
This only applies to grain crops though, as most OSR is Hybrid then farm saving of seed is not really a viable option as it will lose some of its characteristics if memory serves me correct, this comes in 25kg bags.
Sugar beet is a whole different thing altogether, all is produced in mainland europe and bought in as part of the contract through British Sugar, it comes in boxes known as units which I believe is 100,000 seeds