As far as I know, gynecomastia is not listed in the "Indications and Usage" section of Tamoxifen's product information. However, there are many, many drugs that are currently being used to treat conditions other than those they were originally made for.
Tamoxifen is a well established medication for gynecomastia, although it tends to mainly be used (or tried) by specialists (endocrinologists). Whether it helps reduce/reverse gyne seems to be related to the onset of the gynecomastia. The earlier it is given after the onset of gyne, the better chance it has of reducing tissue that is still in an active phase of growing. According to my endocrinologist, it is particularly good at improving the pain/tenderness associated with gynecomastia. If that is someone's main complaint, it is definitely worth a try, according to research.
In addition to the estrogen antagonist, Tamoxifen, anti-estrogen medications (Arimidex, Letrozole) have also proven beneficial for some early gyne sufferers. They don't work for all, but they have been shown to work for some. As my doctor told me, all you can really do is to try one of these meds and see if it helps or works. They are not for long established, stable gyne.
I don't know about the NHS, but typically, meds such as this are NOT prescribed by primary care physicians in the USA. They are usually only prescribed by specialists, probably because it is the specialists who are most aware of the newest research and using certain medications for secondary afflictions. Since these meds might be considered unproven by some, and expensive by many, I would not be surprised at all if the NHS would refuse to prescribe medications such as Tamoxifen. That's their business....denying care and denying costs. I'm sure they do it well, just as the death panels will soon in the USA.
With socialized medicine coming to America through ObamaCare, who knows, drugs like Tamoxifen might not be approved for gyne much longer by physicians in the USA, either. When the government begins regulating how doctors treat their patients, and removes decision making from the doctors, it will not be surprising at all that as little money as possible will be spent. With this type of healthcare, the patient is no longer the priority. The only real priority is how much money is being spent, and that will govern the care that is approved.