Hi, i'd like to install a power shower on the cheap!
(snip)
i've a combi boiler thingy so the imersion is not needed.

You are getting confused by terminology here.

1. The term "power shower" refers to a system that use a pump on your stored
gravity fed hot water system. You don't appear to have such a system, so
this is not possible, although the wiring described would be suitable for
such a shower.

2. The term "electric shower" refers to a system which heats water
instantaneously. This takes a lot of electricity and needs huge cables. An
immersion heater is 3kW. A shower around 10kW. The supply intended for the
immersion will be simply inadequete for this.

3. You say you have a combi boiler. This should provide an excellent shower,
vastly superior to an electric shower. Why are you wishing to install an
electric one?

Christian.