> This gives money to richer voters because the higher your earnings, the more tax saved.

a rise of the zero rate equally effects anyone earning above the new zero rate threshold (regardless of how much they are above it [1])

a full time minimum wage employee earning ~£14,000/year gets exactly the same reduction in tax from this specific increase as an engineer on £90,000/year

[1]: once you get above £100,000/year your personal allowance goes down (ultimately to zero)