The important thing to appreciate is that the name on the prescription is the person with legal responsibility for the prescription.
Therefore although a consultant may advise a medication, if the GP doesn't feel competent or comfortable to prescribe and accept responsibility for the prescription they are under no obligation to prescribe it. Shared Care is entirely optional for GPs.
Shared Care requires the agreement of ALL parties. This includes the patient, to undergo monitoring blood tests for example.
Normal practice is for the consultant to start and stabilise the medication initially, and once stable the GP takes over the prescription.
Most shared care agreements are a written document, outlining the resposibilities of the consultant, GP and patient.