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Reform would have held just 15% of last year’s donations under proposed £100,00 cap, analysis shows
(www.theguardian.com)
Mmm...an important caveat there is that that effect would happen if one considers the impact on the UK as it exists in the absence of such hypothetical rules. I think that you'd find that if you put a cap in place, you would shortly thereafter discover a bunch of lawyers have produced financial structures designed to effectively work around said rules.
For example, there's a limit on how much one can contribute to a political party in the US, but one might contribute to various political action committees or to various non-specific-candidate-centered NGOs or so forth. I don't think that there's any limit on how much one can personally spend promoting a candidate.