Need a part P assessment (£500) before you're able to apply to the elecsa competent persons scheme register (another £370+VAT online). They then send an inspector to 'your regular place of work' (how they do that with self employed plumbers working from home I have no idea) to inspect your installations, H&S proceedures, complaints proceedures, paper work on previously done installations (12 mouths previous work done without part P????), your Public Liability Insurance and your books (17th editon, building reg's, etc)
Turns out, even if you have part P registration, if you are a Plumber fitting an electric shower in a bathroom, because bathrooms, kitchens and outside the property have been deemed 'special zones' you can only fit a replacement shower 'like for like' but to install a new electric shower and run the 10mm electrical cable to feed the power via a pull switch, straight from the CU (as it should be) you have to be a qualified 17th editon electician.
So you either:-
find a good electrician you can trust to turn up at the job and use them to install all your electrics, all the time (bang goes your profit).
Take time out to become an electrician AS WELL.
Tell the customer the work needs to be notified to the authorities before work can start and it will have to be inspected at conciderable cost to the job (bang goes your profit and the job, to a larger Plumbing and Heating firm with their own electricans on staff).
Or You can keep turning electric shower installation work down and if it's to be included in a FULL bathroom suite installation, slowly go out of business in the process.