I can see why you are so confused.
When I first read your post title I assumed it had been suggested/agreed that you were to have a hysterectomy because of the severity of the adeno and so why was he suggesting medical treatment prior to doing the op? But I'm wondering now if that decision has not yet been made on his part. After initial diagnosis the order of treatments is usually painkillers/hormone treatment then the last resort, hysterectomy. This is NHS protocol that perhaps he is following, even though with severe adeno the hormonal treatment is unlikely to have any overall benefit as it aims to give up to 12 months of relief before symptoms all usually kick in again as your hormones get back to 'normal' (which are never normal in endo and adeno). But as you have indicated this would only (perhaps) relieve pain from periods - heavy bleeding, contractions etc. - and not the pain you get all the time. With advanced adeno you will have substantial fibrous masses in your uterine lining that no medication will take away. You will have a thick and 'heavy' uterus.
The taking of any medication is a joint decision between doctor and patient and Prostap (Lupron in the US) does have some documented serious symptoms. It seems clear to me that after the 6 months of what might be a terrible time you will end up having a hysterectomy anyway, which is where I think your thoughts are? In an ideal world we would all have a beautiful uterus, precisely balanced hormones and a pristine pelvis. But we know it doesn't work like that and treatment options for these conditions is a based on a risk assessment depending on severity, age, documented side effects of treatment and outcomes etc. Treatment options must be jointly considered but the ultimate choice must be for you to make as the owner of your body as long as it's an educated and reasonable one, which going straight for a hysterectomy would be with confirmed advanced edeno. You can't be forced to take a medication that would give only short term relief, if at all, and has documented adverse side effects. Your decision then would be how to protect your bone health and other issues arising from hysterectomy and keeping your ovaries would be the usual recommendation if they are healthy. Oestrogen levels would be much reduced but you could potentially have endo recur and would monitor any symptoms knowing they are no longer from adeno. It is unlikely though with properly excised endo followed by a hysterectomy. I think you will have looked at my post on postmenopausal endo/hysterectomy.
This might be helpful in making your decision from Mayo Clinic, a reliable source:
'What are treatment options for adenomyosis?
Effective treatment options for adenomyosis are limited. Although treatments may be effective in some women, the benefit often does not last and women seek alternative treatment options.
Treatments can include:
Hormone treatments including birth control pills, progestins (oral, injection or IUD) or GnRH-analogs such as Lupron
Uterine artery embolization
Unlike fibroids, the adenomyosis cannot easily be shelled out or removed. The only way to remove the adenomyosis is to remove the uterus.
The treatments listed above frequently do not provide adequate symptom control. If a woman opts for a hysterectomy, there are minimally invasive techniques and the ovaries are typically left in place so they can continue to produce hormones.'
I sense you already know what you want and should trust your instincts. Ask your consultant:
Why he suggests Prostap with advanced adeno as documented evidence suggests it will have no long term effect and is associated with adverse side effects that you want to avoid?
Does he think you will ultimately have a hysterectomy anyway?
Does he agree that it would be best to go straight for that in view of the severity of the adeno found and the severity of your symptoms all the time which would not be resolved by Prostap even short term?
Does he agree that you should retain your ovaries and definitely not have oestrogen therapy instead due to its association with reactivating endo?
I hope this might have made things a bit clearer? x