Successful survival strategies for academic departments threatened with closure

• Once you are threatened with closure, it is probably too late to fix your problems.
• Increase enrollment of new students with marketing.
• Increase retention of students with better teaching and extracurricular experiences.
• Seek donations.
• Find a new source of revenue.
• Unionize. A union contract can force cross-subsidies from money-making departments to money-losing departments instead of closure. However, this will not work if there are no money-making departments.

These decisions are all about money, and there's no easy way to get money.

A more detailed answer: https://ep3guide.org/toolkit

This is, in fact, the second time that Leicester's maths department has faced these kind of threats. The previous time (in 2016), they backed down following a petition and other objections organised by a variety of mathematicians.

Check with whatever accreditation outfit your school uses. I was once at an engineering school and the engineering faculty really thought that they could teach the "math their students needed" and would really liked to have gotten rid of the math department. But whatever accreditation body they used to have an accredited engineering program insisted that they have a real math department staffed by real mathematicians.

So the answer might be "if you close your math department, you'll lose your accreditation." And who wants to send their kid to an unaccredited school?