How can a manufacturer of a shower unit dictate what a home owner does with there fixed installation? Oh I cannot buy this shower because I need a switch within the bathroom. Currently it is outside. Damn!Pick any make or model you like do a quick search for installation instructions see what they say. e.g. Mira Sport 9.0kw give us. http://www.mirashowers.co.uk/onlinecatalog/pdf/1175283_w2_d_mira_sport_iug.pdf Note the wording of bullet point 6 under the section Electrical on page 2 which states local isolation must be provided. Whereas bullet point 7. says a 30ma RCD is only recommended. The small drawing clearly shows a local isolation point separate from the CU.
Doc H.
manufacturers instructions should guide you on how to install and use their equipment and all they should say is the installation must comply with BS7671.
If the manufactures instructions say this and it is not required as in BS7671 then it should be disregarded.