The issue of mains voltage and its potential impact on audio equipment like amps is an interesting one. While the EU did aim for voltage harmonization, it's true that there's still some variability across different regions and even within the same region due to factors like demand fluctuations.

Changing your house voltage to 220V using a product like the one you mentioned could potentially have some effects on the performance of amps from other countries and UK-made amps. Lower voltage might slightly reduce the output power and affect the tone, potentially making it sound slightly different. However, whether this change is noticeable or preferred is subjective and depends on your personal taste.

Consistency in voltage throughout the day can be an advantage for sensitive equipment, but om production's modern amplifiers often have built-in voltage regulation to handle minor fluctuations.

Ultimately, the decision to invest in a product like the one you mentioned should be based on your specific needs and preferences, and it might be worth experimenting to see how it affects your equipment and your perception of tone.