alone are responsible for ensuring that the shower as installed now meets current standards. As someone said, it's no different to you swapping out a gas fire and failing to ensure that the new installation meets current gas safety standards. Simple things like knowing what you need to do for an RCD not to be mandatory can trip you up. Also as stated, sizing the cable isn't simple. There are "rules of thumb" which means most electricians can pick a cable size for most common installations without needing to look things up. Cable doesn't have A current rating, it has a selection of current ratings - which one applies depends on the installation conditions (clipped exposed on a wall gives a higher rating then embedded in insulation), and protection measures (it's higher with an MCB than with a fuse as the fuse provides "coarser" protection). Even if the new shower is the same rating as the old one, you could still fall foul of this. The original installation could have been wrong, the loft could have been insulated since it was installed, someone may have replaced with a larger one without checking ... But you are responsible for making sure that the installation is correct when you leave it. So while there is nothing in the building regs to stop you doing it, you would be well advised to seek help. Either work with someone who can take care of the electrical side, or perhaps get yourself on a course