Water pressure and flow available at the shower's cold feed matter just as much as the electrics. An electric shower can only heat the water that reaches it; a poor incoming flow rate limits performance regardless of how high the kW rating goes, since there is simply less water for the unit to raise in temperature.
A thermal cut-out is a safety component, designed to break the circuit if the unit overheats for any reason, whether that is scale build-up, restricted flow, or a fault elsewhere in the unit. A cut-out that has tripped and stayed tripped, or one that trips repeatedly, is a common reason a shower stops working even though the heating element itself is fine. Trade buyers restocking cut-outs are usually replacing a component that has done exactly the job it was designed to do.
This is the practical case for softening in the parts of the UK where mains water runs hard: not as a lifestyle upgrade, but as protection for the heating elements, appliances and pipework that hard water otherwise wears down faster than it should. A softener does not fix existing scale damage on its own, but it stops new scale forming, which changes the maintenance picture on a property considerably over the following years.
Speccing the right option for a job comes down to three questions: how many outlets need hot water, how often do they run together, and what electrical supply is actually available. An instantaneous unit rated for a decent flow rate can pull a serious amount of current, and not every property has the spare capacity on the consumer unit to support it. A cylinder spreads that load differently, drawing steadily rather than in a single high spike, which is often the more practical answer on an older supply.
Serious DIY buyers and trade buyers speccing a softener for a client both need to think in terms of resin capacity matched to household size and water softener supplier UK hardness, not just the headline price of the unit. An undersized softener will regenerate constantly and burn through salt; an oversized one is money spent on capacity that is never used.
Signs worth watching for on a job include a shower or tap that has visibly lost pressure without any obvious blockage, a heating element or immersion that keeps tripping or under-performing, white scale flaking off fittings when they are removed, and appliances needing more frequent descaling or repair than would be expected. Individually, any one of these could have another cause. Together, in a property in a known hard-water area, they point fairly clearly at scale.