1) It is unfair that we are the only part of the UK not to enjoy this part of free at point of use healthcare.
2) 89% of prescriptions in England are issued for free already. Expanding that to 100% would allow the NHS to save on the bureaucracy of administering various "exemptions", collection of money and fraud detection.
3) As highlighted in government material about raising the old age exemption from 60 to 66, charging for prescriptions can lead to people forgoing prescriptions to save money.