Where does U for voltage come from?
The best reason I've heard is to avoid this: -

V = 2 V (which of course is meant to say "voltage = 2 volts")

U = 2 V sounds more sensible after all we use a different symbol for current (I) and also amps. Voltage is a bit on its own - we wouldn't say "amps = 2 amps" or "current = 2 currents".

It seems to me this is the sensible reason for choosing U over V but having said that I never use "U"! Maybe I should?