Hi, sorry you're putting up with this!
Ideally, the GP should refer you to a specialist gynaecologist who should put you on the waiting list for a laparoscopy. This is done under GA and is quite a minor procedure - they stick a camera into your abdomen through a small cut in your belly button, to take pictures and samples for analysis to confirm diagnosis. Then they should discuss treatment.
There are 2 main problems: firstly, some GPs know very little about endometriosis, are very unwilling to refer you on for things like this and will try to tell you that it will go away of its own accord. You might need to be very persistant in asking for a referral. The other problem is that the majority of gynaecologists are not specialists in this condition, and a specialist is what you need. It is very worth pushing to see a specialist if possible.
There are various hormonal treatments available, ranging from the contraceptive pill to much stronger stuff. THey have varying effectiveness depending on how advanced the illness is (if it is mild, the pill might be a good option, but if it is very bad, even the strongest stuff is unlikely to have much effect). You can also ask to be referred to a pain clinic for advice on effective pain relief.
The most effective treatment is surgical excision, but relatively few surgeons perform that in this country. It is worth trying to find one who can.
Many surgeons will perform laser treatment (keyhole, under GA) but it is often not terribly effective, and an inexpertly- performed attempt can even make the problem worse.
It is worth finding your local support group, as they know about what's on offer in your area and are in the same boat as you, and will make you welcome. You can go to a meeting even if you don't have a diagnosis - plenty people do.
Try these sites: this one with good info
this one on finding a consultant in your area
and here to find your local group
Hope that helps.