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.
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.
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...