Hm, don't Brits refer to the rest of Europe as 'the continent'? So, from continental point of view, they are 'the isles'. What's strange or offensive in that?
Anyway, in Polish language there are a lot of such terms, because it's considered poor style to use the same term all over one article or piece of TV news, so they would probably use all the names they can think of, even 'Albion' etc. It gets quite kitschy sometimes and it's even laughed at, for example there is a legendary term for skiing 'białe szaleństwo' (white madness).
Nobody refers to Australia as an island in Poland, as it's considered a continent.
Probably to do to the deep self shame and jealousy to the British nation. Superior Economically and Culturally, therefore emphasize the Island nature of the nation thus making Poland a greater Nation.
Who Cares what they are Superior at, if they live on an Island, surrounded by Wet, Cold Sea, full of Fish, Seaweed and other Stinky objects, ugh, Disgusting!