it isnt
facilitating subjects are so picked because of their breadth. The idea being that if someone didn't really know what they wanted to do, they wouldn't put themselves down a narrow path by picking very specialist courses, and would take a few facilitating subjects that would open up a good range of degree courses they could pursue. This happened to coincide with people's ideas on "hard" and "soft" subjects (though there were a few fairly academic subjects that were not considered facilitating subjects) so it became very misunderstood. Universities have dropped this term because it was turning people away from certain subjects, I don't remember the details.
If you know you want to do a politics or IR degree at university, A-level politics makes a lot of sense. If you're reasonably sure of the area you want to go in to, you don't need to worry about facilitating subjects.