Mains in the UK is generally very clean and well regulated. Messing around with mains filters/conditioners/cables will lighten your wallet, but do little esle.
It is very cheap and easy for an amp manufacturer to include very effective filtering on the mains input to an amp, and therefore they do. So before you go and spend hard earned cash, a simple test will see if you have a problem.
Select an unused input, turn the volume way up, put your ear close to the tweeter. Snap crackle, pop and buzz is RFI, a steady hiss is background noise from the amp. You wont be able to fix the hiss (thermal effects in the amp) but if you have snap crackle and pop then a mains filter MAY improve things.
If you don't have audible RFI noise, then you don't have audible RFI noise, and no amount of filters/conditioners/cables will improve it.
Just as an observation, having lived in the US for a few years in a relatively rural location where the house was fed by overhead cables and a transformer on a pole outside, the mains was all over the place. Having some form of conditioner in these circumstances may make sense. IMO the fad for conditioners is driven by products designed for markets where the mains is very different to the situation in the UK.