Because schools are run as businesses, it is not particularly uncommon for courses to be pulled at the last minute because there aren't enough students (same as uni courses). And because some schools 'headhunt' students from other schools, this is what can happen. The worst part (other the upset to pupils and families) is that in a time of huge shortages of specialist teachers, there are A level teachers with no groups in one school, and groups with no specialist teacher in another.
But this is how modern UK govts want education run. I don't remember when it started, but I do recall Thatcher talking about 'excess school places' in the 1980s, not realising that families move around the country, and that if you move into an area, your child needs a school place somewhere vaguely near.