If you are fitting a new shower and new wiring you must get a qualified electrician to do the wiring to comply with part 13 building regulations - as for which order - I would get the plumber in first - ideally get both in at same time.
Plumbers and CORGI people etc. can do a qualification in this which allows them to do work related to their trade in areas now covered by the regulations. This is called the defined competance scheme
This is specifically so as you don't have to have an electrician and a plumber just to fit a power shower.
Get a plumber and make sure he's qualified under "part P" If he tells you you need a seperate electrician go and get another plumber who is qualified
Jake is correct I can wire in showers, boilers, heating systems and control's aswell as fused spurs under my CORGI Part P certificate. So you only need a qualified plumber who is registerd to self certify minor electrics and plumbing installations.