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