I personally would not be offended at being called a 'Brit'. I wouldn't (and I think most people wouldn't) normally use it in the first person as in 'I am a Brit' unless in a very informal joking type way, more likely in the second or third . . . 'He's typical Brit' etc. Actually, I don't really know of anyone who would say 'I'm a Briton' That evokes images of a battle scarred warrior protecting his village from the invading Viking hordes :-)
Another Brit here reporting that "Brit" should be accepted and is not at all offensive. It's true that it's slightly informal, but it's more commonly used than alternatives like "Briton" or "Britishman".
Please, as a British female how do I say that I am British? I assume that it is not the same. Incidentally I am also quite prepared to call myself a Brit.
The masculine form is "Brytyjczyk", so here in Instrumental: "Jestem Brytyjczykiem".
The feminine form is "Brytyjka", so here in Instrumental: "Jestem Brytyjką".
Strange we don't have a single noun for this. "Brit" is common but definitely slang. We would actually say "British person" in most contexts. But in this case "I am British" is much more natural. I imagine that the Polish sentence "Jestem Brytyjczykiem" is very rarely used.
Because British is an adjective and you don't use an article before a lone adjective (rarely there might be an implied noun)
but why it is adjective and not noun, in another task there is "an Italian eats bread", and there it is noun, where is the difference between them?