A rash? Have you ever been tested for allergies? I know you're trying no gluten and no dairy, but there may be something else in there that you're allergic to. Do you have the funds to go all organic? Some people are allergic to preservatives and/or artifical dyes, or even artifical sugars. Or real sugars for that matter.
I think I would look my GP in the eye and say, "Are you not giving me adequate medical attention because YOU think I'm too fat? Would you refuse to treat a person because you thought they were too black or too Jewish?" There have been studies done that have found that the more overweight you are, the less medical care you get from most physicians. Some will even refuse to treat fat people for knee problems or diabetes and tell them that they have to lose weight first. That's like saying "You're too fat to deserve medical attention: I hope your knee pains you terribly and your untreated diabetes kills you." If losing weight were so easy, then why isn't everyone thin? Lord knows about 1/3rd of all Americans are on a diet at any given time, and yet we're still the fattest nation on earth. Why aren't all those diets working? How come the diet industry (and media) always manages to blame the abyssmal failure rate of diets (95-97%) on a lack of personal willpower? If we accept as fact that the entire human population, minus 3-5%, has this total lack of willpower, do you not think it's better to design a treatment that doesn't require this missing willpower? After all, "Askin' ain't gettin." You can tell people all day long to have willpower, but if they're not disposed to have it, then they won't have it, and no amount of guilt-tripping can make any difference. And if fat is bad, unnatural, a modern epidemic, etc., etc., etc. BS, then why are there so many images of extremely fat women from the prehistoric days? The appearance of fat cavewomen seems to indicate that "fat" may be normal and nautral and part of the overall human experience. Try reading "The Obesity Myth" by Paul Campos.
So your liver's not functioning properly (by the way, I've never heard of fat making your iron levels to be low); did your doctor actually give you some medicine to try and compensate that, or did she just dismiss you as needing to fix it yourself by losing weight? Given your pain and diarrhea, I'm frankly surprised that you are overweight, as many people lose a lot of weight in similar circumstances. I daresay you're not eating much now anyways, so I don't know what "they" think you can do about it.
Did you see that Sarita's problem may be her thyroid? Have you had yours checked lately? They can be hard to find, apparently, AND they can cause weight problems that won't go away. Also, hormones are another thing that can make your bowels go crazy and make you large. Have you had a complete workup of those?
Given your current medical state with your bowels, I think the last thing you want to do is try and diet, given that you are probably not getting enough nutrients now due to lack of eating and, especially, due to your D. Try seeing if you can find a doctor that's sympathetic to people of weight. I was on a board for overweight people once, and I think someone mentioned there being a list (maybe on the Fat Acceptance website?) of doctors who treated people based on their symptoms, not based on their weight. This was an international organization, so if they have such a list, UK doctors will be on it. Also, you may find it useful to go to some of their meetings and ask around for doctor referrals.