It should be a matter of shame, not pride, to the British that they followed a jingoistic US into a needless war in Afghanistan which achieved nothing but much senseless death.

It was.

The 2003 protests against the war in Afghanistan were the biggest in UK history. Approximately 1 Million people (1 in 60 of the UK population) made their way to London to protest, and hundreds of thousands protested in other cities on the day.

https://en.wikipedia.org/wiki/15_February_2003_Iraq_War_prot...

What makes you say that wars are unnecessary? You don't get to see what would have happened in the absence of a war. Keeping some countries like Iran in check by restricting their arsenal, or weakening their economies and military capabilities seems absolutely necessary for world peace for example.