The gains aren't massive — certainly not something any human using the site would notice (about 20ms faster to render pages for the average mobile phone user).
Still good to remove unncessary dependencies, though.
Not on an individual scale, no, but when you consider the millions of visits and views they get per year it adds up from a global / zoomed out point of view.
I'm sure with some math you can make a calculation on how much bandwidth and energy was saved with this One Clever Trick.
A government is the perfect place to look at this "zoomed out" view.
Your citizens an average rate of pay of Y per year. Some of time they save by getting stuff done on a government website will be put into getting more economic work done. If the citizens do 10 billion pageviews of gov.uk per year, and you shave off 20ms, thats 27 human-years of work per year. So it is certainly worth making this optimization, even if it takes a few people a few months to figure out how to remove jquery.
Still good to remove unncessary dependencies, though.