How ISO differs from ANSI
ISO is the keyboard standard used across the UK and most of Europe. Three things differ from the ANSI layout common in the US: the Enter key is tall and narrow rather than wide and flat, the left Shift is short with an extra key beside it, and there is an additional key next to Enter on the home row.
That extra key beside the left Shift reports as IntlBackslash, and it is the single most useful key to test on an ISO board, because it does not exist on ANSI at all. If your operating system is set to a US layout, that key often produces nothing recognisable even though the switch is perfectly fine.
The layout setting matters as much as the hardware
A browser reports two different things for every key press. The code is the physical position on the board and does not change with your language settings. The character is what the operating system decided that position should type, and it changes completely when you switch between a UK and a US layout.
This is why an ISO keyboard on a US layout puts the at sign and the double quote in the wrong places, and the hash key types a backslash. Nothing is broken. Changing the operating system keyboard layout to match the physical board fixes it, and the readout below the tester makes it obvious which of the two is wrong.
If the code is right and the character is wrong, it is a software layout problem. If the code never appears at all, it is the key.
Testing a UK board specifically
On a UK ISO keyboard, the key to the left of Enter carries the hash and tilde legends, and the key beside the left Shift carries backslash and pipe. Press both. If either reports nothing while its neighbours work, the switch or its trace is at fault.
The pound sign sits on Shift and 3 on a UK layout. Because it is produced by a combination rather than its own key, a failure there is nearly always a layout setting rather than a hardware fault.