If all you’re concerned about is overvoltage tripping, then as Glyn points out, your supplier is under a statutory obligation to maintain the voltage within the prescribed limits. As a first move, you could try threatening to hire a calibrated monitor and claim the cost from them - it might show that you mean business.
But if you’ve considered that unit for other reasons, then I must say I’m not sure about some of the advertising claims for those devices. I’d consider very carefully especially the “energy saving” parts, because you’ll probably find they won’t stack up in practice, e.g. when boiling a kettle, for example, running at a lower voltage will just take longer, and it’s possible that more heat will be convected and radiated away - thus lost to ambient instead of boiling the water - due to the longer time taken. In the most general terms, it will only save energy if the control system you presently use isn’t working effectively.