Sorry, Ceri, I'm very weak on this subject.
Though you may want to see an endocrinologist instead of a GP. Most GP's do not run into trans people in the normal course of events and may be hesitant to prescribe HRT in the first place, but if you have lupus, then they may be extremely resistant to anything which mucks about with your hormonal system.
An endo, especially one who has a history of working with trans people, may be more open to allowing HRT to proceed especially if there is closely monitored blood levels and, of course, a letter from a therapist that indicates that this is a medically necessary course of action.
-Sandy