There's also a historic point to note here. Traditionally, British shows are made in much shorter runs than in the US.
E.g.) The Office was two series of 8 shows, whereas something like 24 or Lost will run to more than 20 episodes in a 'season'.
This means that no show would ever run long enough to warrant being called a 'season' which (as I think someone above has said) derives from the US TV habit of planning programming around the year's seasons, hence the 'Fall season' or 'Spring season' or whatever.
I don't think the TV announcers here would ever use 'season' in describing a set of programmes, I can even remember instances of very long-running shows being described as 'series' when they come to an end.
This is also the reason why there are so few episodes of 'Fawlty Towers' - it originally ran for just two series, so there are only 12 episodes.
Interestingly, and without wishing to bore you all, I think there's a school of thought developing that suggests both sides have got it wrong, in that UK series are too short to engage the viewer and US seasons are too long to keep a show fresh.