John Rumm wrote:

Guessing at some of the terms:
"magnetothermal switch" = MCB
"30mA differential switch" = RCD

Agreed.

the 2 x 16A seems odd - perhaps it does mean 32A.

It might, perhaps, mean a linked 2-pole MCB, with the intention that the
equipment is supplied from 2 phases of a 3-phase system (common in some
parts of Europe). Or even with the intention of the MCB breaking both
phase & neutral of a single-phase feed (in which case just use an
ordinary single-pole MCB in the UK).

That would also tie up with a 6mm^2 CSA cable (assuming the distance
is not excessive). No idea where the 25mm comes into it.

Or did it mean 6-core 2.5 mm^2? Lord knows why though. Or a 6 mm o/d
cable? Is there a rating plate on the damn thing stating voltage,
current, power or VA, no. of phases, etc.?

The "The resistance of the eart hold much not exceed 3ohm" seems an odd
requirement to specify on a circuit which is also RCD protected. (might
make more sense on a circuit that relies on EEBADS protection only - but
then the 3 ohm figure would be too high to provide fast enough
disconnection of the MCB (a quick look at fig 3.4 in Annex 3 of BS7671
suggest Ze would need to be = 1.5 ohm for a 32A MCB). I think you can
ignore this bit since you have the RCD (and since you will probably
exceed this requirement by a good margin anyway)

Agreed: ignore and follow BS 7671 / the OSG.

To get this right, you will need to go back to basics and read the
actual current loading or power rating off the plate that should appear
somewhere on the bath.

Quite.

"this electrical line must uniquely serve the bathtub" sounds like a
suggestion that there be an equipotential bond run back to the CU.

I read "electrical line" as referring to the final circuit supplying the
appliance (together with its CPC). IOW it's just saying that this
bathtub must be fed from a dedicated final circuit, rather than being
hooked into the light with a BC adaptor...

--
Andy