If you are counting Gift Aid towards your pledge it is recommended to calculate your pledge amount based on your pre-tax income. If you are not claiming Gift Aid or any tax benefit then it is recommended to calculate your pledge amount based on your post-tax income.
Here’s a summary of UK tax treatment of charitable donations. As I understand it, donations are sometime deductible, and sometimes eligible for gift aid, but never both.
GWWC has a somewhat complicated treatment of gift aid; you can count gift aid towards your pledge, but