Hard water contains higher concentrations of calcium and magnesium ions. It is common in many parts of the UK and is a major contributor to limescale in kettles, taps and heating appliances.
Important distinction: reducing dissolved contaminants and softening water are not necessarily the same job. Choose technology based on the outcome you actually want.
If your main problem is limescale
Look for a system specifically designed and tested for hardness reduction if that is the desired outcome. A drinking-water filter may improve taste or reduce particular contaminants without making the water “soft”.
If your goal is drinking-water quality
RO can reduce many dissolved substances, but the exact performance depends on the system and maintenance. See does RO remove limescale?
Check your local water first
Water hardness varies by supply area. Your local water supplier publishes water-quality information, which is a better starting point than assuming all UK tap water is the same.