the mirror found in the Chettle Hoard, Dorset was an early period mirror (La Tene influence) but found in quite a late context.
There is also this link (not an archaeologist so don't get too excited about the Celtic bit...)
http://www.celticmirrors.org/
Thanks, Petar. It's sad to read that these artefacts are being sold into private hands and will never be seen in museums.