So, despite the existence of "GBP" which has unambiguous meaning worldwide
on any documents written in any language, someone goes and invents "UKP",
which on the day they invented it meant nothing to anybody other than
themselves. Why??

Because they can.
Because they want to.

I have done it. So what ? Not everyone is au-fait or cares about ISO
codes anyhow, including some of the banks judging by the state of
their FX software.
Dave