Hi
@retseldrib,
If you specifically search for 'uk mains frequency specification', your top hit is likely to be the following:
"The GB mains frequency is nominally 50Hz. National Grid is obliged by its licence commitments to control the frequency within ±1% of 50Hz so it can fluctuate between 49.5Hz to 50.5Hz. However the normal operational limits are 49.8Hz to 50.2Hz."
As your Rega motor is AC synchronous, it is the mains frequency that governs the speed (250RPM @ 50Hz), whereas, voltage governs torque. Ideally, the frequency should be spot on 50Hz, with the voltage in an optimum range that provides enough torque to overcome friction, whilst at the same time allowing the motor to run as quietly as possible. Typically, external supplies regenerate a near perfectly stable 50Hz (often quartz referenced), plus a preferred optimum stable voltage. Some, like the Naim Armageddon, don't regenerate the frequency (à la Linn Valhalla, etc.), rather they are intended to optimize voltage across the two phases of the motor for an optimum blend of torque vs. phase angle.
The Heed units mentioned above have an output stability spec of 0.1% frequency shift with a max. 1% THD of the sinusoidal waveform. Other makes likely offer similar performance, however, I'm not familiar with the ones that list such. With the Rega Neo, the specified speed control step size of 0.01RPM would indicate a similarly high specification, by necessity (likely better).
P.S. Keep in mind that, at 60Hz, the Premotec motor will turn at 300RPM. It wouldn't take much effort to calculate what ±1% of 50Hz would equate to as speed fluctuation range.
P.P.S. Ok, call me pedantic. For every 0.5Hz deviation in mains frequency the motor speed will change by 2.5RPM; therefore, the permissible ±1% of 50Hz will allow a 24-pole AC synchronous motor to vary between 247.5 to 252.5RPM, which equates to 33.0 to 33.7RPM at the platter.