So I don't think the UK evidence shows what you claim it does. The smallest UK spoilt ballot error I can find is larger than the winning margins in many US places.
Finally, here's [4] one analysis of voting tech versus ballot spoilage rates that seems to reach similar conclusions of all the papers I scanned over. Hand edited paper is about the worst way to get valid ballots of any method in this study.
[1] https://www.votenone.org.uk/spoilt-ballot-results-2015.html
[2] https://www.votenone.org.uk/spoilt-ballot-results-2015.html
[3] https://www.theguardian.com/uk/2012/nov/19/spoilt-ballots-pc...
[4] https://www.hamilton.edu/news/florida/Klinkner%20Analysis.pd...