It looks like it's the GOVUK Frontend JS. Most of it is probably not necessary.
That being said, the site feels instantaneous to me, and scores 95/100 on PageSpeed Insights, with the only oversight being the render-blocking Google font which adds 750ms to initial render, but should be cached on subsequent navigations. Perhaps your browser isn't caching it, which would explain the sluggishness.
200kB of JavaScript (50kB gzipped) is pretty lean for modern websites. React.js on its own is around 130kB, for just the library.
However, 209kb of javascript? I'm no JS snob, but goodness gracious what in the world?
It's a collection of hyperlinks and a spash of CSS. And it's actually fairly sluggish.