The till uses Unicode and the £ symbol is stored as number 163.

This statement doesn't make sense. Unicode isn't an encoding, UTF8/UTF16/etc... are. So while £ has the codepoint of U+0163, the UTF8 encoding would be 0xC2 0xA3 and the UTF16 encoding is 0x00A3.

If the printer was expecting Windows codepage 436, it would have displayed the UTF8 as ┬ú (assuming the font supported it). If it was sent as UTF16, it probably would just skip the NUL byte and only show the ú.

All speculation unless you know what encoding is transmitted on the wire, and what encoding the printer expects.