If its an electric one and is the first time you have fitted an electric one you need both. If the wiring is already in and up to teh relevant standard you just need a plumber.
Check the size of shower you currently have against the shower you want to install, if they are rated the same, ie 8kw, then it should be straight forward for a plumber, if anything has to be changed within the wiring, then you need a plumber who is qualified to part p standard, or an electrician, the wiring needs to be certified with building control, the plumbing doesnt.