Confusingly, in British English series refers to both an indiviual season and the collection of all seasons (or to the show itself, in the abstract).
For example, here's the Series history section of the Wikipedia article on Grange Hill:
Grange Hill was originally conceived by ATV comedy writer Phil
Redmond, who first approached various television companies with the
idea in 1975, unsuccessfully. In 1976, he managed to sell the idea to
the BBC, and the children's drama executive Anna Home gave the series
a trial run of nine episodes, the first being broadcast on 8 February
From the start, the series caused controversy for its real-life, gritty
portrayal of school life, rather than the more idealistic school dramas that
preceded it. Redmond has said that he wasn't really able to start pushing the
boundaries until later series. This led to Redmond being summoned to lunch
by BBC bosses and forced to agree that there would be no further series
unless he toned things down. Grange Hill's highest profile period was
undoubtedly the mid-late 1980s. One of the most famous storylines during this
time was that of Zammo McGuire and his addiction to heroin. This storyline
ran over two series (1986–87) and focused on Zammo's descent into drugs and
how it strained his relationship with girlfriend Jackie and friend Kevin. The
show's other favourite characters during this period were Gonch and Hollo
played by John Holmes and Bradley Sheppard. During his time at the school
(1985–89) Gonch took part in many moneymaking schemes, most unsuccessful.
There was a comedic element to the duo's relationship that worked well with
viewers. Script editor Anthony Minghella, who worked on the series for
several years during the 1980s, later won an Academy Award for Best Director
for the film The English Patient in 1996.
During the 1990s, Grange Hill did not receive the same media attention it did
just a few years previously. The teachers were now equals in the narrative
with their personal lives taking up almost as much time as those of the
pupils. In 1994, two characters were introduced with disabilities, Denny
Roberts (Lisa Hammond), who suffered from dwarfism, and Rachel Burns
(Francesca Martinez), who had cerebral palsy. Both characters were presented
as "one of the gang" and hated any special treatment because of their
circumstances. This prompted the BFI's 2002 publication The Hill And Beyond
to comment that Grange Hill had perhaps become politically correct. Beginning
on 4 April 1993, to celebrate Grange Hill's 15th anniversary, the first
fifteen series of Grange Hill were repeated during CBBC's Sunday, and later
Saturday morning slots on BBC1 and BBC2. The repeats ended with Series 16 in
1999. Interest in Grange Hill renewed in the late 1990s and the series
celebrated its 20th anniversary with the introduction of sinister Scottish
bully Sean Pearce (Iain Robertson), who carried a knife and slashed the face
of a classmate. Cast member Laura Sadler, who was heavily involved in this
storyline, died after falling out of a building in June 2003; four years
earlier her Grange Hill character Judi Jeffreys was killed after slipping and
falling out of the window of a burning storeroom in the school.
By 2001, the series was almost entirely issue-led and the decision to tackle
the subject of rape upset some parents. But when Phil Redmond took over
production of Grange Hill in 2003, his plan was to get the show back to its
roots and the issues were toned down as Redmond skewed the show towards a
younger audience. In early 2006, it was announced a film of Grange Hill was
to be released in late 2007 focusing on the lives of former pupils but has
not yet appeared.
Grange Hill returned on 14 April 2008 with its final series, including a
return of the original theme music. Series 31 returned to BBC1 after the 2007
series was shown exclusively on the CBBC Channel.
However, the US English usage season is also widely understood, and can be used to distinguish between the two concepts without sounding "American" (in the same way that sidewalk for pavement would).