Wiring in a shower

I can confirm that you would need an a qualified electrician if the electric shower unit doesn’t already have existing wiring. If the wiring doesn’t exist then it needs to be connected to the consumer unit and appropriate type of wiring needs to be used. Note that the size of the supply cable is a key factor. In some cases 6mm² cable will be adequate for a shower rated up to 8kW. For showers above 8kW it may be necessary to install 10mm² cable. In order to comply with Part P building regulations a competent installer familiar with the requirements of the current I.E.E regulations must assess the proposed circuit in order to select the correct cable size.

Yes

Yes, In a domestic installation, amy work in a bathroom must be done by a regsistered contractor. A previous answer stated that competency and knowing the regulations will suffice, IT DOES NOT. It is a legal obligation that you are bound to, make sure they are registered domestic installers.