[joshuaissac]

I did not know until now that the other 20% could be reclaimed by the donor as well, if they paid 40% tax.

If the donor donates that amount as well and we loop infinitely, the final amount received by the charity would be exactly 1.5625x the originally-donated amount (assuming that the HMRC allows arbitrary small fractions of a penny to be claimed).

Explanation: if you donate an amount x, the final amount would be the infinite series x + sigma(0.25*0.2^(k-1) + 0.2^k, k=1, k=inf), which is the sum of two separate geometric series, so for each one, we can use the formula a/(1-r), where 'a' is the first term (0.25 for the first series, 0.2 for the second) and 'r' is the ratio between each term (0.2 in both cases).