JAPAB I think that's an important point for discussion and I was actually thinking that I should clarify that in practice this doesn't mean disagreement in every case.
I agree with you up to a point but not fully. If we take a couple of examples you gave, donating a kidney should obviously only be done with the permission of the person donating in the case of a live donor. If the donor was deceased it could be a bit more complicated as others such as relatives might be the ones giving consent on the person's behalf.
But if we take the example of tattoos, again obviously I would agree that no-one should be tattooed against their will. However, presumably you would also say that an adult should have the right TO get a tattoo if they wish, whereas in Islam tattooing is forbidden altogether. So if I knew a Muslim who wanted to get a tattoo I would disagree with that and try to convince them not to do it by explaining the Islamic ruling on it, whereas in my experience most people in the West will accept that and believe it should be the person's own choice whether they do it or not.
It's interesting that the example we focused on is circumcision, as with regards to other issues the general principle is that Muslims should not make any permanent body modifications without a medical need, even if they want to do that.
So I guess the point I'm trying to make is that although we would agree on many issues in practice, the actual principles and ideas that inform our respective attitudes are very different. You are correct in saying that the difference in our perspectives is unlikely to be brought out unless there is an actual difference in practice.
Let's imagine for example a hypothetical case where a woman's parents were forcing her to get married to a man that she disliked, and she was distressed at the prospect. Presumably we would both agree that she should not be forced. But unless someone actually enquired as to our reasoning behind it, they wouldn't realise that my opinion is based on the fact that the Prophet Muhammad (peace and blessings of Allah be upon him)instructed us not to force women to get married, whereas you would probably mention arguments based on human rights and individual choice.
I guess that's one of my reasons really for getting involved in discussions on this forum, I think that because different cultures do agree in practice on many issues, people tend to assume that the same principles and reasoning underpin those agreements, which can then lead to frustration and confusion when areas of disagreement such as circumcision become a topic of discussion.