How to choose a water softener
Assuming you have established that you need one — hard water above roughly 200 mg/l CaCO₃, and a problem you actually want solved — the choice comes down to sizing, regeneration type, salt type and the quality of the installation.
The last of those matters more than the brand.
Sizing
A softener is sized on two things: how hard your water is, and how much of it you use. The unit's capacity is expressed in the volume of water it can soften between regenerations, and that volume falls as hardness rises.
Undersize it and it regenerates constantly, using more salt and more water, and hard water breaks through at peak demand. Oversize it and water sits in the resin bed for too long between regenerations, which is its own problem.
This is a calculation, and any installer worth using will do it rather than reaching for whatever is in the van. Give them your actual hardness figure and an honest account of household size.
Metered or timed
Metered units regenerate based on the volume of water actually used. They are more efficient on both salt and water, they cope with variable demand, and they are what you should buy.
Timed units regenerate on a schedule regardless of use. Cheaper, and they waste salt and water when you are away and run out of capacity when you have guests.
The price difference no longer justifies choosing timed. Twin-tank metered units go further again, regenerating one tank while the other stays in service so there is never an interruption — worth it in larger households.
Block salt or granular
Block salt is clean, easy to handle, and comes in packs sized for the compact units that dominate the UK domestic market. It costs more per kilogram.
Granular or tablet salt is cheaper per kilogram, comes in 25kg sacks, and requires you to lift a 25kg sack.
The machine dictates this choice more than you do — block salt units take block salt. If you have a free choice, weigh the running cost against whether you want to be carrying sacks.
The installation details that matter
A bypass. Not optional. Without it, servicing or removing the softener means no water in the property.
An unsoftened drinking tap. Standard UK practice, and the reason is the sodium that ion exchange adds in proportion to the hardness removed. Every competent installer does this without being asked. If a quote does not mention it, ask.
A proper drain connection. The softener discharges brine during regeneration, and the connection must have the correct air gap so waste can never siphon back into the supply. This is a backflow protection requirement, and it is one of the main reasons this is not a DIY job.
Somewhere sensible to put it. Near the stopcock, near a drain, with room to get the lid off and lift salt in. Under-stairs cupboards and garages usually work; the back of a fitted kitchen usually does not.
Questions to ask the installer
What hardness figure did you size this on, and where did it come from?
Does the quote include the bypass and an unsoftened drinking tap?
Where exactly will the drain connection go?
Are you WaterSafe or WIAPS approved, and can I check the register?
What is the workmanship warranty, as distinct from the manufacturer's?
What will servicing cost, and how often?