In the UK, "Series" refers to the programme itself. So, Doctor Who is a series; Coronation Street is a series; CIS is a series. In addtion, what in the US is called a season is also called a series. So, we talk about the current series of Strictly Come Dancing; the fourth series of Blakes Seven in which everybody died etc. People at the bus stop will be talking about the new series of Footballers Wives starting etc. So, although season and series are technically interchangeable terms, for the man on the street in the UK they probably aren't.
I don't mind if people refer to Doctor Who's series as seasons, but I will call them series because it's a British show (I might even call them seriels, too!) For US shows, I'll refer to them as seasons. So, I've just finished watching season 3 of Dexter and I'm looking forward to the new series of Doctor Who.