Getting into math ten years after a BA in English
If you want to go into math, should you get a BSc?

I'd say "yes". It often is quite doable to pick up a new subject on your own if you have academic experience, but English and Mathematics are so far apart that I'd doubt there would be a lot of synergy. Mathematics has its own way of thinking, which is probably picked up best by going through an undergrad degree.

Should you go into math?

Enrolling for a degree is of course a rather strong decision. In my experience quite a few of those who start anew later in their lifes drop out rather early. Maybe taking an online course first could both be a good preparation if you go through with it, and helpful to figure out if you really want to it.

Can you get in somewhere?

I don't know about your country, but both the UK and Germany often have places reserved for mature students, which have fewer formal requirements. So in those countries, getting into a decent program would be quite doable.

A final comment

For me at least mathematics is great fun. If you believe you'd like it, and you are willing to put in the effort, give it a try.