If your hair behaves in a hotel abroad but not in your own bathroom, the water is a fair suspect.
Most of London, and a good stretch of southern and eastern England, sits on chalk and limestone. Rainwater filters through that rock and picks up dissolved calcium and magnesium on the way to your tap. That is what "hard water" means — not dirty water, just mineral-rich water. It is perfectly safe to drink. It is also quietly working against your hair every time you wash it.
What the minerals actually do
Two things, mostly.
First, calcium and magnesium react with the cleansing agents in shampoo. Those agents are designed to lift oil and lather in soft water; in hard water a chunk of them binds to the minerals instead and forms a dull, insoluble residue — the same scum you see on a shower screen. Less of your shampoo does the job it came to do, so the instinct is to use more, which leaves more to rinse out.
Second, those minerals don't all rinse away. Over weeks they build a fine film on the hair shaft and across the scalp. That film is the real culprit behind the symptoms men blame on their shampoo: hair that feels rougher than it should, looks flat rather than shiny, tangles more, and never quite feels properly clean. Colour-treated and lighter hair tends to show it first, but everyone accumulates it.
What it probably doesn't do
Here the honest answer matters. Hard water gets blamed for hair loss and for structurally weakening hair, and the evidence for that is thin. A few small studies have looked at whether mineral exposure reduces the strength of a hair fibre, and the results are mixed at best — some found a difference, others found none. There is no good evidence that hard water makes your hair fall out.
So treat the buildup as a surface and manageability problem, not a health one. That is a more useful way to think about it, because a surface problem has a straightforward fix.
Clearing the buildup
Ordinary shampoo is formulated to remove oil and dirt, not to lift bonded mineral deposits. To shift those you want a clarifying wash — one built to dissolve residue rather than just clean the surface — used periodically rather than daily.
The Detox Shampoo is made for exactly this: it clears the film of product residue and mineral scale that a daily shampoo leaves behind, so your hair goes back to feeling like hair rather than something with a coating on it. Once a week is plenty for most men, twice if you use a lot of styling product or train hard. The reason not to do it every day is the same reason not to over-cleanse your face — strip the scalp too often and it simply produces more oil to compensate, and you have traded one problem for another.
A few habits help between washes. Rinse for longer than feels necessary; most men stop early. A final cool rinse helps the outer layer of the hair lie flat, which is what actually reads as shine. And if you want to deal with the water itself rather than its effects, an inexpensive shower filter reduces the mineral load reaching your hair, and a plumbed-in softener does it properly if you own the place and want to commit.
When it isn't the water
Buildup makes a scalp feel coated and occasionally a little itchy, and clearing it usually settles that. But hard water is an easy thing to blame, and it is not the cause of everything.
If your scalp is persistently flaky, itchy, sore, or shedding noticeably more than usual, that is not a residue problem and no shampoo is the answer. See a GP or a dermatologist — those are things worth having looked at properly rather than washing at.
For the far more common complaint — hair that has gradually stopped behaving in a city bathroom — the cause is usually the least dramatic one. It is the water, and the fix is to clear what it leaves behind and stop there.