Some folks here seem to be confusing insulation thickness with creepage specification. Your insulation specification may call for 8mm minimum separation for creepage, meaning that the shortest path between the conductors concerned along the surface of the PCB and/or the surface of the components must be at least 8mm. But that does not mean you need 8mm think insulation. So a PCB much thinner than 8mm can meet the requirements, in principle. You would still need to remove the copper within about 4mm of the edges of the board on each side to meet the creepage requirements.
But why bother going to the trouble of getting your PCB quality-controlled for thickness and dielectric strength and insulation tested, and having an extra certification hurdle to jump, when it is much easier to remove the copper from the other side of the board under the mains traces?
As an example of meeting requirements using thin insulation, the ADUM6401 series of digital isolators achieve 5kV RMS UL 1577 isolation with just 0.017mm thick (minimum) insulation (and 8mm creepage clearance across the package).