Or pass new legislation. The government tried to use existing environment legislation to ban single use plastics by listing them among other toxic substances. That that didn't pass judicial review isn't actually that surprising (depending on how the original law was drafted).

The alternative is just to amend that law to add additional powers to ban bioaccumulating and/or biomagnifying substances, of which microplastics are just one example.

It's just more work, obviously, than using an existing legislative tool.