Britain: no standard week in law, and the 28 days may already include the bank holidays
Britain has no statutory standard week at all. The Working Time Regulations 1998 cap the average week at 48 hours over a 17-week reference period, and even that can be individually opted out of in writing, something few of the other systems here let an employee do on their own. There is no legal figure to divide a salary by, which is why the calculator opens on 37.5 hours as the usual full-time contract and expects you to replace it with your own.
The leave figure is the one that catches people. Statutory entitlement is 5.6 weeks, which is 28 days for a five-day week, and employers may count bank holidays towards it. So an English contract offering "28 days including bank holidays" and one offering "20 days plus the eight bank holidays" are the same entitlement described two ways. That is why this page shows 28 days of leave and zero public holidays rather than adding the two together, which would overstate British time off by nearly a third and understate the hourly rate accordingly.
There is no statutory overtime premium in Britain. Any enhanced rate comes from your contract, and pay must simply not fall below the National Minimum Wage across the hours worked. That last clause does real work: unpaid or flat-rate overtime that drags your average hourly pay under the minimum wage is unlawful even if your headline salary is comfortably above it. Running your actual hours through the calculator is the quickest way to see where you sit.