Installation of the electrical circuit for a new shower is 'notifiable' under Regulation 12(6A)of the Building Regulations 2010 (as amended). Anyone can do the work but it must be 'signed off' by a 'registered competent person' OR a 'registered third party' OR a 'building control body'.
So, as long as you use a 'registered competent person' (such as an NICEIC member:
http://www.niceic.com/find-a-contractor/find-contractors ) they'll do all of the paperwork. (i.e. complete a BS 7671 electrical installation certificate, giving a copy to you and to the local building control authority).
You might find it better to seek out a suitably-qualified plumbing and heating contractor though, as that will save you having to employ both a plumber and an electrician. Any member of CIPHE (
http://www.ciphe.org.uk/ ) should be able to do the work for you, either 'signing it off' themselves or being able to call upon someone else ('a registered third party') who can.