Electrical Regulations are a mess at times. There is no requirement to have a registered electrician fit what is merely a replacement - like for like.
Sometimes, plumbers fit these, simply because the electrician doesn't want to involve himself in the water connections.
In your case, I would strongly advise having an Electrical Installation Condition Report carried out. Otherwise known as a Periodic Inspection. This would test the shower installation as well as the rest of the house. It should be done every 10 years for an owner-occupier, but every 5 years for a rental.
Whenever I'm selling, I do this as a matter of course. It gives the buyer's lawyer one less thing to throw at you ;o)