This is absolutely the case. There are other areas where 'standards' aren't used because of accessibility reasons; a prime example being the HTML5 built-in vali

This is absolutely the case. There are other areas where 'standards' aren't used because of accessibility reasons; a prime example being the HTML5 built-in validations. They just aren't reliably accessible enough across browsers/platforms. The GOV.UK Design System is extremely well thought out, there's reason and sound arguments behind every decision - and the best part is that it's all developed in the open.