It's not illegal, but it should be.
Background: The Bristol University Christian Union has apparently banned lone female speakers - unless they're with husbands.
Okay, so that's the oversimplified version. In reality, female speakers have been disallowed for longer, but the vote to allow them caused a senior member to throw a huff and resign, and so a compromise was reached to appease the fundamentalist aspects of the organisation. Yes, bizarrely, banning female speakers at events unless they co-present with a husband was progress.
But why are they allowed to do this? Isn't it discrimination?
Yes, it is. But it's legal.
As a religious organisation CUs and their overarching governing body the UCCF are exempt from equality legislation. As a centrally funded group, they also don't tend to be ratified student societies - so are exempt from any legislation that governing students unions have. This means they don't get funding, and usually get a bum deal with rooms, but actually it doesn't affect them too much. CUs aren't necessarily obstructed from doing what they do, they're just not given the helping hands other student groups get. So there's nothing to stop them doing this.
But why should there be? It's not illegal for them to discriminate how they like. No one is forced to join a Christian Union, in many cases they're not (even though they like to pretend) the only Christian group in a university. The only alternative to this being perfectly allowable is for it to not be allowed, and that requires us to have over-reaching laws that restrict what people think, what they believe, and how they go about assembling to share that with other like-minded people.
Even my hard-core-liberal-atheist ass sees that as a line not worth crossing.
Although this is, admittedly, clever semantics rather than meaningful, it's not that it's legal, it's just not not illegal. There's no law saying that discrimination is allowed, there's just no law telling them it isn't allowed. The UCCF already wholeheartedly and openly discriminates on its membership and speakers; it can only allow those who accept it's ludicrously fundamentalist doctrinal basis - something I've mentioned before. It would seem intuitively wrong to say they must not discriminate against atheists and Muslims by abandoning this. It would be like telling an LGBT group that it needs to advocate and accommodate straight people more - it would defy the point.
So the CU exists. And when you sign up to it you take their shit in exchange. If you don't agree with the doctrinal basis and fundamentalism that plagues it, you're free to go elsewhere. If you're stuck thinking you must adhere to CU regulations you disagree with just because you signed up, then that's your own weak-minded problem. This is a problem with all prescriptive belief systems and organised groups that bind people together, but I don't think there's any need to make this illegal because that would mean banning all collaborations! Let them have their rules, and let them pick and choose as they like. We can't force them to do otherwise under any circumstances, because it's our freedom to mock them that would be at stake too.