As Mohair says, there are several grounds on which an employer can in principle discipline you for something said in the pub: there is no general sanctuary offered by licensed premises! Furthermore there are rules whose details I don't know, about when a social event to which colleagues are generally invited can be considered a work-related event and therefore directly subject to HR scrutiny.
Relevant to this case, though, HR might be concerned about a couple of things, and I'm bearing in mind that what you've quoted might not be the worst of what was reported:
That whoever was taking down the notes was alarmed/distressed/harassed by what you were saying: perhaps they are a pregnant Muslim themself, they are considering becoming one, their loved ones are, or they are claiming that your antipathy was so palpable as to disturb them. If so then the fact you've distressed them outside of the office doesn't automatically make it none of HR's business that you're distressing your colleague. It's not automatically their business even if you really have (unintentionally) distressed a colleague, though. So it's not at all clear from the limited evidence (a) whether this has happened, (b) whether HR's informant has claimed this happened, and (c) whether it's appropriate for HR to be involved if it has. But bullying of colleagues outside the workplace, or even unintentional distress caused in post-work social events, is a potential legitimate concern for HR. So this doesn't constitute a legal opinion that you can't appeal their decision to take action, merely that I don't believe there's any absolute bar on them taking action where bullying or other harm is suspected, and that this might at least be claimed based on what you've said.
That there's a group of people who go out after work to complain behind their colleagues' backs about how annoying it is that those colleagues happen to be pregnant women, Muslims, or members of other groups subject to relatively high risk of discrimination. I don't say you have any intent to discriminate or that you actually have discriminated, but you can't really bandy about someone's legally-protected characteristics as a criticism of them even in jest. Regardless of where you are when you put about this (mildly) negative view of pregnancy, your group's shared views of pregnancy as expressed in these remarks will travel with you back to the office, and that's why HR has to care at least a bit.
I sort of think the first example does this: "her pregnancy makes me angry" is a direct (even though joking) criticism of a protected characteristic. The second one might in the right context be a perfectly constructive suggestion, and so in the absence of context looks all right to me as far as it's written. But I observe that there are different ways of saying "it'd be nice", and since we're both Brits and not strangers to sarcasm, I think we know that it can range from a completely sincere statement of mild preference, to a passive-aggressive rebuke, to outright mockery. I am not in HR and neither do I adjudicate this stuff, so I'm certainly not saying they're right (or wrong) in these cases. I'm just saying this is the light in which you should consider what you're saying when you're in the pub with colleagues after work. Your opinions of them relate to the workplace regardless of being expressed outside it.
And sure, this can all get very ticky-tacky with offence found where none was intended or taken. But that's why there's laws about it, because the UK considers even unintended issues of this kind to be genuine and serious problems. So you may have to apologise and move on.
Public swearing is "interesting" in English law. There was a time when it would pretty much automatically be considered intentionally offensive and hence a public order offence if a copper takes a dislike to it. Your employer of course can take an interest in the possibility that a group of employees is out on the town threatening Her Majesty's Peace and the very basis of civilisation as we know it.
But there was a case in the High Court in 2011 where Justice Bean ruled that the audience to the swearing (police officers and a group of youths being spot-searched for drugs) could not possibly be genuinely offended by the use of what I assume to be the f-word as general punctuation ("---- this" ... "you won't find ---- all" ... "I've already ------- told you"). The conviction was overturned, but it's really your call whether you want to try to defend your language in the face of HR telling you not to use it in the presence of colleagues who don't like it.
