- power shower which uses water heated by your hot water system. Be aware that more than five minutes under a power shower can use as much water as a bath! - electric shower which heats its own water. These are good as you'll still have a hot shower even if your boiler fails, but they have less grunt as flow rate is affected by temperature.
needs to be fitted by someone with the right electrical qualifications and knowledge.
most electricians will be familiar with fitting an electric shower.
most plumbers will not have paid the fees and bought the electrical test gear required to authorise them to install a new circuit and a new electrical device in a bathroom per Building Regulations. Some heating engineers will.
When you phone, ask which Competent Person Scheme they are a member of, and since when. You can then verify it on the Scheme's website.