Nope you are quite right. There's no regulation in BS7671 to say 'when installing an electric shower for Mrs Pete999, there must be a form of local double pole isolation, with 3mm gap blah blah'.
537.3.2.1 suggests a circuit breaker is suitable means of isolation, although further guidance on this, can be obtained from Table 53.4 note (5). There's others, which I can't see the point of arguing over. Only TN supplies, think your up a gum tree with TT, but don't quote me.
134.1.1 'Good workmanship etc etc. The installation of electrical equipment shall take account of manufacturers instructions'.
I believe most, if not all domestic shower manufacturers recommend a double pole 'switch' ceiling mounted, or wall mounted in an adjacent room.
Seems like the manufactures see it as a good idea, who am I to argue?