This was a UK paperback, somehow the protagonist was on Venus with two or three companions. They discovered that 'the jungles' are populated by tribes of wild humans transplanted from Earth.
Venus has long been ruled by Martians who stomp about in tripods and hunt the humans for sport. The protagonist infiltrates a Martian city and learns there is a large population of tame humans who co-operate in hunting the wild humans. The Martians 'prefer the taste'.
The wild tribes speak in a working class English and are descended from Cockneys. The collaborators are middle class English and spend all their time jockeying for positions of power and plotting to get each other assigned to 'the fattening pens'.
The protagonist is hiding and witnesses Martians feeding by draining captives' blood. He unites the wild tribes in concerted attacks.