They use both words but when they say different series they mean a different show - like Lost is one series and Heroes is another. They say season to mean what we mean when we say different series. I'm all for resisting the americanization of UK english.
I use both for each region, I've always called a US show Seasons and a UK show Series, 24 Season 5/Spooks Series 5. I would never call a UK show a Season though, it's just wrong.
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