Why should any political donations other than a standard annual party membership fee (say £25) be permitted?
All other options allow wealthy people disproportionate political influence.
I sympathise with the sentiment, but: this would give a largeish party of 100000 members a budget of £2.5 million per year. Let's say in a typical year there are public elections of some variety in areas containing 10 million households. I think a political party can reasonably expect to be allowed to deliver an election-address leaflet to every household. That gives them 25p per leaflet to get them printed. I'm not convinced that's feasible.