UK Mains Frequency Limits
Amber Switches are not needed to protect the loads but are exclusively designed for "islander" renewable low power generators with no energy storage. 

The spec here is load regulated generator frequency which lowers in RPM or Hz under high loads and rises above norm, with low loads.

If you are not the owner of the grid and live in the U.k. where the freq. tolerance is adequate, there is no purpose to be served here.

Unless you are participating in "load shedding" for reduced rates,  there is no need to do this.

The Amber Switch has a hysteresis of 0.25 Hz which can be set to a very limited range value for 50 or 60Hz


  Therefore your requirement is unnecessary.


Keep in mind heavy motor loads are 5 to 8x rated full power when switched ON unless custom low surge designed. This is far greater than a 5% frequency change that occurs very slowly or even a 1~2 swing as noted in the UK. Here in Toronto, we can sync our OCXO's to the grid.(lol)