In the UK, eligible citizens have access to free medical prescriptions. Does the NHS have special deals with drug companies and if so what are they?
Like other national drug providers they do make deals with the drug companies. These big buyers representing a large market have a bargaining power better than small ones, lowering the unit price for them.
Usually these deals aren't public to protect the drug companies' interests (and the buyer agrees to this for a lower unit price). If unit prices were disclosed for contracts between buyers and drug companies, others without a deal could use this information to strike a better deal (than they could get without this information).
Sometimes the buyer and the company don't get into an agreement and there is no other company that could sell a product for the same use. This has happened in some countries offering free healthcare in the case of a very rare condition and an extremely expensive drug for that (e.g. Nusinersen).
According to this overview document about NHS procurement by Specialist Pharmacy Service, the reality is a little more complicated. The NHS is divided into 10 regional groups managed by the Commercial Medicines Unit (CMU). The groups manage requests for tenders and procurement while using frameworks awarded and managed by the CMU. Depending on the drug sometimes the groups do joint procurement.
I'm not 100% sure that drug procurements in NHS aren't public information, but since I failed to find any I think they aren't (feel free to prove me wrong if I am).