"Ed Sirett" said:

Bad 5mm - 6mm - 7mm - 8mm good.

I spoke to a couple of bath manufacturers before buying last year.
They claimed it wasn't quite as straight forward as Ed describes.
They claimed that the forming process tends to thin the bath wall more as
the wall thickness increases.
So the best option is a 5mm bath with a secondary coating. Ideal Standard
make one whose name eludes me.
I actually went for an "Aquanited" model from Aquabeau - guaranteed 25
years.

There were one or two quality issues, but structurally it seems solid.

Phil