Hi and welcome. I don't mean to shoot you down but from the look of what you've written, you are not allowed to do this work.
1) you must be part 'p' registered to fit a new circuit to the consumer unit
2) it will be in a special location as it's a shower which means it must be notified to labc (building control)
3) all new circuits contained within the shower room must be protected by an rcd
4)you don't have the necessary test equipment to test the new cable (even new cable can have a fault)
i dont mean to be nasty but you need to get a qualified electrician in before you harm yourself or worse, someone else.
