GPs do not get commision from consultants, unless there are some very dodgy doctors around doing this (totally unethical IMO and don't think the medical council would be too impressed either). Who is chosen usual depends on the GP's previously experience of the consultant - this could be from feedback from other patients, treatment of previous patients, maybe hearing the consultant speak at a medical meeting or knowing them from college or having worked with them. There is a directory of consultants published every year and new consultants setting up in private practice often send out a letter to GPs outlining their experience, treatments offered etc.