Lol, that's for legal migrants

https://www.gov.uk/government/publications/application-for-change-of-conditions-of-leave-to-allow-access-to-public-funds-if-your-circumstances-change

You can apply online for a change to your conditions if your financial circumstances change and you:

• have permission to stay on the basis of your family or private life
• have applied for permission to stay on the basis of your family or private life
• have permission to stay with a close relative with protection status as a child
• hold a British National (Overseas) visa
• have permission granted on any other immigration route and you want to apply on a discretionary basis because your circumstances are particularly compelling

Note the permission thing.