The UK's Competition and Markets Authority has found against a number of companies including old favourites Advanz/Cinven. The product is prochlorperazine 3mg dissolvable or “buccal” tablets.
Pharmaceuticals: suspected anti-competitive agreements
The CMA is investigating alleged anti-competitive agreements in the pharmaceutical sector under Chapter I CA98 (case reference: 50511-2).
gov.uk/cma-cases/pharmaceut...
Press release
CMA fines firms over £35m for illegal arrangement for NHS drug
The CMA has fined firms over £35 million for an illegal arrangement in the supply of important NHS prescription anti-nausea tablets.
gov.uk/government/news/cma-...
Inevitably, there will be appeals.
This is just to reassure you that thyroid patients are not the only ones having issues with drug pricing.
T3 Scandal - Watchdog Revealed Drug Firm overcharged NHS by 6,000% rise for T3 - Guardian Article
Liothyronine CMA fine company
ADVANZ FINED! Fines of over £100m dished out for unfair pricing of liothyronine 
But I wonder if patients were prevented from being prescribed this treatment because of the high cost? That would be interesting to know. Somehow I doubt it.
A good question - I do not know.
Have a look here:
bnf.nice.org.uk/medicinal-f...
You can see that 3mg buccal tablets have a bizarre range of prices from £11.08 to £50.27. I suspect that many GPs would at the least look at reducing prescription quantities at the higher end - if they are aware.