Many of the government services available via GOV.UK are designed to be accessible and usable even when JavaScript is disabled. These services are not designed as SPAs (and all the better for it).

For example, here is a demo of an accessible autocomplete drop-down list that uses JavaScript. When JavaScript is disabled, the list becomes a standard HTML drop-down list.

https://govuk-location-picker-demo.herokuapp.com/

Here is the generic code for the autocomplete on GitHub:

https://github.com/alphagov/accessible-autocomplete

And here is a blog post about dealing with a large amount of form inputs:

https://accessibility.blog.gov.uk/2019/04/08/accessibility-l...