Absolute balderdash. Every last word.
By all means share info with farmers, even if they are actually your competitor in supplying a product and in order to enable it to be sold ever cheaper. But why allow that data to be passed on to any other related industry? No other industry lets their production costs be known to their customers or rivals or boast about how little they can sell their product for while still making a profit.
Price is a function of supply and demand as long as the market works properly as you must believe it to. In real life it is open to all kinds of abuse and manipulation. Part of which is to encourage just enough surplus to keep you producing at the average marginal cost of production, no more.
My neighbour is my friend, not my competitor. Your attitude is merely another symptom of what is broken in agriculture. Together, we stand. Divided, we fall.
You have made a couple of good points - yes, we are unusual in that aggregated data is published which is not done for many other sectors. We're all very different - at the risk of contradicting my first sentence in this post, if I can produce goods for 10% cheaper than the industry average then there's my competitive advantage if they want to nail us down to a tiny margin. However, "Continually pressing prices lower" is an ongoing pressure applied to all goods - the retailers just want to buy cheaper than their competitors so they can make more margin. They couldn't give a toss about whether half of UK farmers go bust as long as the shelves are full. Our share of the retail price is so low as to be insignificant for most foods anyway. The retailers won't stop when UK Ag plc is at a break even point based on benchmark data - they will keep going as long as they can get a supply from somewhere. It's when they run short that they will start preaching about "working with our suppliers for a sustainable food chain."
You have another good point - there's no such thing as a perfect market. That's just an economic theory. Non perishable futures quoted commodities traded globally e.g. wheat or maize, get quite close to that but there's freight supply/cost, protectionist tariffs or subsidies etc that skew the supply/demand equilibrium. Milk, meat etc are very susceptible to the exercise of market power not helped by the long production cycles. There's no value added beyond a bland commodity captured by farmers here anyway unless they are running niche shops, cafes etc.
The retailers already have our cost of production data - look at the "cost plus" deals for milk that are helped by Promar's benchmarking (a condition of the contract).