The programme, The Celts, was produced by the BBC not the BBS. The book
of the series is freely available in both hardback and paperback.
Like the other BBC series on English it suffers from the BBC's decision
to not use someone who knows about the subject but to use journalists or
authors. In the case of the Celts, it was done by an Irish broadcaster
called Frank Delaney and is couched in terms of Delaney in search of his
roots. It suffers from the fact that Delaney doesn't really belief in the
existence of the Celts today and is full of glib misinformation.
I suggest you try to get hold of the programme series, Ainm a'Ghaidheil,
instead made by real Celts by real Celts - it is in Gaelic but has subtitles
and the views of experts are not bent to fit preconceptions.