I guess that, as a government, they are required to cater for people with accessibility issues (in this case: people who cannot type but need to dictate to enter text).
If you then cannot support the most popular dictating application, that’s not a good look.
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.
If you then cannot support the most popular dictating application, that’s not a good look.