I don't recall seeing anything on my tax return which laid out how much of my income was taxed at 20% and how much at 40%.
It won't be shown on your tax return.
But it is detailed in your Self Assessment calculation.
This shows how the income left after deduction of Personal Allowance had been taxed.
But then I think I give enough to charity to effectively reduce my income below £50k.
I suspect you are getting further confused. Most charitable (Gift Aid) payments do not reduce your taxable income. They do however increase the amount of your basic rate tax band, which in turn can reduce the amount of higher rate tax payable.
For example if your taxable income is £54,000 and you make Gift Aid payments of £5,000 (gross amount including basic rate tax relief) then your taxable income is still £54,000. But your basic rate band will be increased from £37,500 to £42,500.
NB. UK figures for current tax year, different if you are Scottish resident for tax purposes.