It's worth noting that ALT+X gives you the default OEM code page for compatibility with DOS sigh whereas ALT+0X gives you Unicode. So typing ALT+0163 will give you £.

>sigh whereas ALT+0X gives you Unicode. So typing ALT+0163 will give you £.

This is incorrect. It gives you an ANSI codepage. On old Windows version it would be a default ANSI codepage, on modern Windows it's a codepage associated with your input language. So if I type ALT+0163 with English keyboard layout I get £ from Win-1252, but the same combo after switching to Russian gives me Ј from Win-1251.

Entering numbers bigger than 255 just causes wraparound. For example, ALT+0835 also will give you £ instead of ₣.