No tax liability but expenses

What happens if your tax liability for the year is nil, but you have expenses that can be claimed, such as children's dentristry or further education.

You don't get any tax relief. Because you have paid no tax. Unlike credits at source which benefit even non taxpayers (e.g. SSIA topups, owner occupier mortgage interest tax relief, private health insurance premium tax relief etc.).

It seems very unfair that a person on a low income pays at least 20% more for medical and dental expenses for their family, as well as college fees than those better off.

I *think* the Green Party had something in their taxation position paper about this, they certainly did about refundable tax credits.