When you complete your new UCAS application you would list your achieved A levels - all of them, even if you are retaking any, or taking other subjects. You would also list the A level you are resitting or taking-as-new in the 'pending qualifications' section. If your school allows you to apply through their school UCAS portal, they can then add their reference and may be willing to add new predicted grades.
However, if you do a single new A level subject privately and not via the school, its unlikely they will want to do this, and without a private tutor, this can make UCAS applications tricky as Unis will want a predicted grade. It is also much harder taking an entirely new subject than just revising/resitting an existing subject - do not underestimate the amount of work involved or how you will manage this without any face-to-face teaching.