I think, but I could be wrong on this, that they buy them in as a package of films rather than individually. It always seems to be 1 or 2 good films and 4 or 5 absolute useless excuses for films that no-one would ever pay money for.
I think the companies have the films on contract for a time period? - as for several years they seem to be shown on just the one channel? I wonder if there are also repeat restrictions/quotas?
Don't know about BBC, but I find I can no longer watch most films on ITV - not because of the ads, but because they all seem to be cut to ribbons, turning 18 rated films into PG.