For example:
In 2022-23 Greg earns £55,000 and he and Sue have two young children and so they have child benefit of £1,788 per year. Ordinarily they would end up paying back £894. But for every £80 of donation by Greg, they recover £18 child benefit on top of the personal tax relief of £20 that he can claim. That’s a total £100 to the charity for a net outlay of £42. An effective rate of tax relief of 47.5% of the donation of £80 in addition to what the charity gets.
If your income is in the range above £100,000 where tax-free personal allowances are eroded by £1 for every £2 of income, your effective tax rate in that band would be 60%, and for every £80 donated your personal tax claim would be worth £30 instead of £20. So, HCF would get £100 for a net outlay by you of £50. Thus, you have obtained 37.5% of relief on your £80 donation in addition to what HCF gets.