I'm not used to reading law in English, so this is all a bit hard for me to parse, but I'm having trouble finding the section that prescribes the addition of ac

I'm not used to reading law in English, so this is all a bit hard for me to parse, but I'm having trouble finding the section that prescribes the addition of accessibility features to all software written for any purpose. Say I am a British person who decides to make a some common piece of software, like a spreadsheet, and offer it for download on the internet. Am I required by law to make it accessible? And if, for instance, the provision of accessibility features would require thousands of hours of redevelopment, does that fall under "reasonable adjustments"? I understand that the moment my software becomes required for someone's job, for instance, there might be a problem, but a few posts up it was implied that making software without accessibility features was in itself illegal in some way, and that just sounds doesn't sound right to me.