Some beefy power showers draw around 40 amps, depending on the pump chosen.
I'm not qualified to give an opinion but here is my view as an amateur .... spurring off the light socket sounds like a problem waiting to happen.
Your pump may only draw 3 amps now but the next owner, in ten years time, may upgrade the unit to something more powerful. If you are going to spur, at least spur off the mains with a 13 amp fcu. Preferably give the shower its own circuit from the CU with a 45 amp MCB.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local