You can use any of these websites to book tickets for any National Rail journey in the UK. You aren't limited to the operator's site for their own trains, and be aware of third party websites because they often charge booking fees, while the websites run by actual train operators generally don't.
I will be contacting these companies soon.
Going through all these websites in quick succession shows that there's actually only about 4 different booking systems which are rebranded accordingly. Each have their own title options off-the-shelf, so perhaps one changing would prompt the others to follow suit. I will be contacting those listed above who only offer gendered titles to ask them what's up.