With apology, I'm not sure I even know what you're talking about. "Thrush" typically means yeast infection of the mouth and throat, but it sounds like you are speaking about a genital infection. (Maybe "thrush" is used in UK for candida (yeast) vulvovaginitis, but I'm not aware of that.)
So for the moment, I'll assume you mean candida vulvovaginitis. There are some strains that are resistant to the standard treatments, but usually it isn't particularly difficult to find other treatments that work. I have never heard of true genital yeast infection in women that persisted for 8 years; that makes me skeptical of her diagnosis. Yeast infection should not interfere with conception or pregnancy, by the way.
I also don't know how to relate this to your herpes history. Herpes doesn't trigger yeast infection, nor does herpes treatment. (Other antibiotics sometimes do that, however.)
As you may know, the UK has a network of genitourinary medicine (GUM) clinics, equivalent to STD clinics in the US but generally more sophisticated than their US counterparts. There are GUM clinics in most large and mid-size cities. With few if any exceptions, the care is the best a person can get for problems like your wife's. By contrast, my impression is that the quality of knowledge about STDs and other GU infections often is poor among GPs and other non-GUM-speciaty physicians in your country. So if you haven't done so, your next step should be evaluation at a GUM clinic.
Good luck-- HHH, MD
Thanks. yes, over here thrush refers to a vaginal yeast infectiona s well in the throat etc.