You cannot refuse to dispense such items:
"Can I refuse to dispense a medicine that I do not receive discount in obtaining and which is not on the Zero Discount List?
Pharmacy contractors, through their Terms of Service, are obliged to dispensed all medicines ordered with reasonable promptness, regardless of whether discount is received when obtaining a product (The National Health Service (Pharmaceutical Services) Regulations 2005. Schedule 1; Part 2;5.(2)(b)). "
says the PSNC website:
Welcome to PSNC- Speaking for Community Pharmacy
I believe you should be able to argue the case for Zoladex to be added to the new-fangled version of the "ZD list", if indeed neither Unichem nor AAH give discount on it; see the PSNC page mentioned above to find out how. Good luck!