In developing their directives, the EU's top-10 list of items to ban was based on a review of what was found on beaches. They surveyed 263 beaches across Europe and counted and classified things by what they were—cigarette butts, etc. From their perspective that was enough for them to be able to take action against that top 10 list.
They also had an impact assessment study, but it looked at multiple perspectives, including some science, but primarily socio-economic...and a number of policy measures and various scenarios. The EU has initiated some scientific work to support the directive and is looking at undertaking further work, for example on microplastics, which wasn't captured in the directive, but is number one in the “dirty dozen” you heard about from Peter Ross.