UK is largely vaccinated and the festival required negative test (which some people undoubtedly faked) or vaccination certificate.
https://www.itv.com/news/meridian/2021-08-26/how-covid-safe-is-reading-festival
The article mentions a 40k festival where 1000 people tested positive for COVID-19, and case fatality rate in UK is now 0.5% as everyone older is vaccinated. Reading was 100k, so ~2k possible cases, which gives ~10 potential deaths from COVID-19.