Probably more oligopoly, but the difference is only academic.But if they are integrated right up to the retail outlet, they can take all the profit from that end, while keeping the farm gate price at rock bottom to squeeze out the competition.
It's called monopoly.
Worth noting there's a consultation running at the mo, on contract structures in the pig trade.
Contractual Practice in the UK Pig Sector - Defra - Citizen Space
This site contains consultations that are run by Defra.
consult.defra.gov.uk