power line induced hum question

Yep, it's 50Hz or harmonics of 50Hz. Which are most commonly strongest at 100Hz with higher harmonics depending on how 'leaky' the circuit that's inducing the hum is and how much gain it's applying.

Waves give the option of choosing 50 or 60Hz simulated mains hum on a lot of their hardware emulations. Not because the emulations hum very much, but because if you use a bunch of them and they emulate 60Hz mains while you've got e.g. guitar amp hum at 50Hz and multiples thereof the result can be quite nasty.

Edited to add - while in the UK the voltage might vary a little, e.g. where I am it's usually around 237V but can drop to 232 or go up to around 245 the AC frequency is rock solid at 50Hz. The voltage is theoretically 220V across the EU, but with a big enough margin of error allowed to mean when that standard was adopted no country had to change the voltage it already supplied. The UK being nominally 240V at the time.