The key points are  consent  adequate assessment including consideration of access to previous records, modality of communication, need for physical examination.
The GMC guidance is given as;
60. Before you prescribe for a patient via telephone, video-link or online, you must satisfy yourself that you can make an adequate assessment, establish a dialogue and obtain the patient’s consent in accordance with the guidance at paragraphs 20–29.
61. You may prescribe only when you have adequate knowledge of the patient’s health, and are satisfied that the medicines serve the patient’s needs. You must consider:
a. the limitations of the medium through which you are communicating with the patient
b. the need for physical examination or other assessments
c. whether you have access to the patient’s medical records.
62. You must undertake a physical examination of patients before prescribing non-surgical cosmetic medicinal products such as Botox, Dysport or Vistabel or other injectable cosmetic medicines. You must not therefore prescribe these medicines by telephone, video-link, or online.
Update July 2015
A new EU law came into effect on 1 July 2015 that requires doctors to prescribe only drugs that are licensed for use where the patient lives – for example, when they are prescribing drugs via online consultations. >> Find out more in section 7 of the MHRA’s document (pdf)