I think it's a token gesture to try and get others on the bandwagon. According to this article: Almost all plastic in the ocean comes from just 10 rivers | DW | 30.11.2017 - http://www.dw.com/en/almost-all-plastic-in-the-ocean-comes-from-just-10-rivers/a-41581484 and other sources, most environmental plastic burden comes from sources where there is a large population with inadequate rubbish collection infrastructure. Hundreds of millions of people, maybe billions, simply chuck used plastic products into the river, or allow them to end up there, unaware that this is a bad thing. OTOH much of the microscopic material is tyre dust and paint dust - neither of which are likely to be banned soon. So the UK's plastic straws are not really a big component of anything but they provide a reference point to generate some publicity and dialogue about the wider problem.
That was my thoughts. Here in the UK we keep banging about recycling, so it will stops scenes like in the Blue Planet. Never watched the programme, and granted we have our own local issues, but all those picture I've seen are of far off exotic locations.