The usual scenario is that a bunch of students decide that they want to be principled and/or edgy and invite someone unpopular and/or unpleasant to speak. The SU says 'no, we don't let our space be used for meetings with Nazis/homophobes/Erdogan enthusiasts/whoever', and now the university (imagined by the law as a kind of parent body of the SU) has some legal duties.
I think part of the 'menace' fought by the Act when introduced was the anti-apartheid boycott of South Africa. Many SUs wouldn't platform pro-apartheid speakers, while student Conservative Party clubs generally took the British government line against the ANC and in favour of the SA government.
In the case I mentioned it wasn't the SU's space, it was the debate club's. It wasn't Durham's SU that has the problem, it was the national SU, and they bused in protestors.
I think part of the 'menace' fought by the Act when introduced was the anti-apartheid boycott of South Africa. Many SUs wouldn't platform pro-apartheid speakers, while student Conservative Party clubs generally took the British government line against the ANC and in favour of the SA government.