Supplementary bonding in bathroomsDoes every bathroom still need supplementary bonding?

Supplementary bonding links exposed and extraneous conductive parts locally, e.g. the metal pipes under a bath to a metal shower, so they stay at the same potential. You'll often see it as a loop of green/yellow between copper pipes, such as at a boiler. Somewhere along those pipes, a main bonding conductor goes back to the MET.

BS 7671 · Section 701: locations containing a bath or shower — starts p.257 BS 7671 · Reg 701.415.2 supplementary bonding, and when it may be omitted · p.258

Supplementary bonding conductor sizes

This depends on what you're linking and whether the conductor is mechanically protected, e.g. in conduit. A single-core green/yellow conductor has only one layer of insulation. The grey sheath on twin and earth is mechanical protection, and an RCD is additional protection, not mechanical.

CPC of the circuit feeding the exposed part = 10 mm²
Link: electric shower (exposed part) → pipes under the bath (extraneous part)
Not mechanically protected → 6 mm² (still 6 mm² if protected)
Exposed part → exposed part, both mechanically protected with 10 mm² → 10 mm²

Electrician's Guide to the Building Regs · 3.3.3 Supplementary bonding conductors · Table 3.3.3 sizes · p.62