I currently work in a noisy office (shouting, explosive sneezing, etc.).
I find this very distracting when I'm trying to do work that requires
concentration, which is most of the time. Raising the issue with
management is unlikely to help. I currently wear Sennheiser HD 202
closed back headphones, which have good sound quality, but don't block
external noise much, and are uncomfortable to wear all day.

Can anyone recommend a better noise-blocking solution? I don't mind
spending a few hundred pounds if necessary.

The reviews for Sennheiser HD-25 headphones suggest they are good at
blocking out noise but squeeze the head very hard?

Would active noise-cancelling headphones help? The reviews I've read
suggest that they are more effective with fairly constant noises, rather
than the sudden unpredictable noise in an office. I would prefer a model
that I could adapt to be powered from a USB port.

Ideally the headphones should not be too garish either.