I was unfamiliar with "Posafilin" until your message, but a quick online search suggests it is a UK trade name for an OTC version of podophillum resin, most commonly called podophillin in the US. As you suggest, it is marketed primarily or solely for non-genital warts.
I try not to "compete" with health care professionals who actually have examined and counseled a patient. But in this case I agree with you and disagree with the doctor.
Podophillin has been used for decades for treatment of genital warts. However, it is possible that the manufacturer of Posafilin is using a stronger concentration of resin; in STD clinics, a 10% (or sometimes up to 25%) concentration is used. One source I found indicates a 20% solution in Posfilin, but I don't know if that is the only version. That's pretty high and could indeed be very irritating to delicate genital skin surrounding the wart(s), especially on mucous membranes or moist tissues, such as labia minor or vaginal opening, the anus, or under the foreskin in men. Indeed, when podophyllin is used in the US, the standard recommendations--including CDC's STD treatment guidelines--advise it be applied by a health care provider, i.e. not considered suitable for self-application by patients. And clearly, according to the labeling you describe, the manufacturer does not want its product used on the genitals. On quick search of the scientific literature (PubMed), I can find no studies of posafilin for treatment of genital warts.
There is a product called Condylox (in the US), which contains podofilox--a purified ingredient extracted from podophillin. It is approved and often used by self-treatment. But it's likely much more expensive than Posafilin. Other available treatments are freezing (in the health care provider's office) and imiquimod (trade name Aldara).
Having said all that, if your warts are on a dry surface such as your labia major, and if you carefully apply the product only to the wart(s), avoid contact with surrounding skin, and dry the product (e.g., with a hair dryer) so it won't smear around onto normal skin, then the treatment likely will be effective and not cause undue irritation. But I would not use it on moist areas.
If you are in the UK, I recommend you get a second opinion at a GUM clinic; or its STD clinic equivalent if you are in the US or elsewhere. As to general information on HPV and warts, excellent information is available from the American Social Health Association (www.ashastd.org), CDC (www.cdc.gov/std), and others. You also can read the excellent article (I wrote in myself!) found in the link "STD Quick Facts and Articles" on the STD forum home page.
Feel free to print out this response and send it to the doctor you saw!
Good luck-- HHH, MD
Thankyou so much for your help, I am seeking a second opinion today, and will also be reading your articles. Thankyou so much!