It depends for what medication. For NFA-VPS and POM-VPS medicines, any pharmacist can write a script or supply the product. For POM-V or human products, no they cannot prescribe - only a veterinary surgeon can do so.
Now, strictly speaking there are no extra qualifications required of a pharmacist who is registered with the General Pharmaceutical Council (GPhC). However, in practice, most pharmacists do take additional training to allow them to prescribe and supply appropriately and responsibly (something they are legally bound to do).
In addition, the GPhC does require that they be taking additional training (Continuing Professional Development, CPD) in the areas in which they practice; so if they intend to prescribe or supply animal medications, they would need to be able to provide evidence that they had received some additional training or education in the area.
Does that help?