One example is the RNIB (Blindness and sight loss charity in the UK) - they use various bits of GOV.UK Frontend on their website, such as the breadcrumbs and accordion on [0]. Many local councils use it too (or a version of it).

Because it's so heavily tested for accessibility it makes sense to use in those kinds of contexts where accessibility is a very high priority.

GOV.UK Frontend can be heavily customised via Sass variables (eg the font stack in [1]) so the components don't have to look like GOV.UK if you don't want them to. GOV.UK Vue pulls all its styling from Frontend so it's as customisable as Frontend is.

[0] https://www.rnib.org.uk/living-with-sight-loss/assistive-aid...

[1] https://github.com/alphagov/govuk-frontend/blob/main/package...