Im not an expert on how UK tax system works, but there generally is only one personal allowance of ~GBP12k, and thats it (no other deductions or credits, which the average US tax payers have a ton of, thanks to special interest groups). So based on that, the effective tax rate seems pretty simple and its just the tax brackets. For UK:
https://gamefaqs.gamespot.com/a/forum/c/c69cd8e5.jpg
So for most people (50k-125k), effective tax rate is 33%. For US:
https://gamefaqs.gamespot.com/a/forum/e/eade61bf.jpg
Effective tax rate is only 7%-11% for the same income, despite the tax bracket reaching 24% for incomes between $95k-$180k.
Sources:
https://taxsummaries.pwc.com/united-kingdom/individual/taxes-on-personal-income
https://www.pewresearch.org/short-reads/2023/04/18/who-pays-and-doesnt-pay-federal-income-taxes-in-the-us/
https://i.imgur.com/XNEdLrJ.png