For heaven's sake, no word is more intrinsically offensive or rude than any other. It relies entirely on context and the appropriateness of the situation, most often both. I'm quite free with my use of c**t, or f**k in company where it is acceptable. Using it in front of my Gran at chistmas or in a board meeting is totally unacceptable. There are times when I have used traditionally racist terms such as 'p**i' when I'm parodying the kind of knuckle dragging attitude, but only in company where I'm 100% certain that the company is intelligent enough to unsderstand the point that I'm making. I would never, ever say it in public, and to do so on film is mind numbingly stupid. In fact it makes me more sure that Harry is not really aware about how offensive it might be.
I think you need to ask yourself, why try to use the 'Brit' argument? It can only mean that a) you have zero concept odf what is offensive, and thus there's no hope for you. Or b) you're fighting to be able to keep saying something that's offensive and that makes you the kind of person that society could do without. So why don't you take option c) which is to just not use it anymore, no-one gets offended, no-one loses anything and society benefits.