There's a good chance that NICE won't be willing to fund these treatments at all. Which is in no way a criticism of them (QALY is the just the most sensible semi-objective thing to base these decisions on) - but is worth highlighting when you say "free healthcare for all".
It's only free for the healthcare considered to be good enough value for money.
That's a hard thing to admit, so people look for a way around it. But at the end of the day somebody has to pay. If you tell the drug companies that the products are too expensive, and ask them to accept lower returns than comparable high-tech, high-risk, talent and capital-intensive industries, you're asking those companies and their shareholders to pay. (Or you're asking them to cross-subsidize those treatments with money from treatments that have a much larger number of patients.) People love that idea because it makes it seem like you're getting cheaper treatments for "free" but all you're doing is hiding the costs. There is a cost, it's now just hidden and unaccounted for. If you told Apple they could only charge $250 for an iPhone, what do you think would happen?
Let the drug companies charge what they want to charge, and let government-funded healthcare programs decide what they're willing to pay to save a particular life. That's the most transparent and ethical thing to do.
Theoretically, it would be possible for states to do this responsibly in a manner that benefited everyone, but if I weigh the extent to which our system allows interested parties to pay for legislation against the extent to which the public is likely to make reinsurance rates a ballot issue, I can't help but put the probability of a responsible implementation at about 1-2% and the probability of "privatize the gains and socialize the losses" at 98-99%.
Come to think of it, the general idea is "If you, as an insurance company, foot the bill for these treatments with long-term benefits for your clients, this will cost $X, but save you $Y and other nearby insurance companies $Z in the future. Therefore, we the association will pay you $z < $Z to do it, and by assumption $Y+$Z > $X, so there should exist values of $z that make this profitable for both parties (anything between $X-$Y and $Z)." The carrot is the payment of $z.
In terms of whether this association could become a price-fixing cartel... It's probably easier to verify compliance when compliance consists of performing a positive behavior (i.e. paying healthcare providers to do certain treatments; whatever the details of the payment, the important thing is whether the treatment was performed) instead of a negative behavior (i.e. not selling extra product at a lower price on the side). It doesn't seem obvious that this would make price-fixing much easier than it currently is.