What is the minimum water pressure a water supplier must provide?

The 7 metre static head standard applies at the outside stop tap — the boundary of your property — and not at your taps. Everything after that point is your own pipework, and that's where most disappointing showers are actually created.

So the practical order is: if the pressure is poor everywhere in the house and always has been, raise it with United Utilities, because it may be a supply-side issue and they have obligations they can be held to. They keep a register of properties at risk of low pressure, they'll test at the boundary for you, and that one measurement tells you which side of the line your problem sits on. If the pressure is poor at one outlet only, or it's got worse over time while the neighbours are fine, the fault is almost certainly inside your boundary and it's yours.

Two things worth adding for Macclesfield. First, don't confuse a genuine breach of the standard with the ordinary consequence of living up a hill: United Utilities are explicit that pressure varies with elevation, and high ground legitimately sits lower in the range without anyone being in breach of anything. Second, the standard is about pressure, not flow — and in the town's older terraces it is very often flow, strangled by a narrow or corroded old supply pipe, that's actually ruining the shower. United Utilities can be entirely within their obligations at your boundary while your bathroom remains disappointing, and in that case the fix is your pipe, not their main.