The responsibility for choosing the medication for use with a patient rests with the prescribing clinician.  Best practice is to negotiate this in partnership with the patient.  There is however a moral and contractual obligation to take into account the cost and effectiveness of the drug and be prepared to account for your decisions if called upon to do so.

GMC (Good Medical Practice 2013)
16. In providing clinical care you must:
b. provide effective treatments based on the best available evidence
18. You must make good use of the resources available to you.
3.  You are responsible for the prescriptions you sign and your decisions and actions when you supply and administer medicines and devices or authorise or instruct others to do so. You must be prepared to explain and justify your decisions and actions when prescribing, administering and managing medicines.

GMS Contract
14.9. Excessive prescribing  
14.9.1. The Contractor shall not prescribe drugs, medicines or appliances whose cost or quantity, in relation to any patient, is, by reason of the character of the drug, medicine or appliance in question, in excess of that which was reasonably necessary for the proper treatment of that patient.
14.9.2. In considering whether a Contractor has breached its obligations under 14.9.1, the Board must seek the views of the Local Medical Committee (if any) for the area in which the Contractor provides services under the Contract