Not terribly clear what they mean at the moment.
I'm guessing that by maths they mean STEM, but what happens if you want to do a mix, maybe maths, FM, economics and history? Or is it going to be a full sixth form which just happens to have a good enough maths department to offer several maths A levels?
Given both the raising of the participation age, and the demographic bulge that's been stressing younger-age school entry points, it does make sense to increase sixth form provision when there's still time to do so in a planned (rather than last-minute) way.
So it's a good thing there will be more places. It's a good thing to champion STEM. We have to wait and see what these colleges (and their entry procedures) will actually be like.
Bloody irritating to link it to BREXIT, though.