I have just moved into my new house and I want to know which shower is best as the water pressure is very poor upstairs.

I currently have a mixer tap with shower which is very poor for getting showers as when you put the hot tap on the cold tap slows down and if someone down stairs puts the cold tap on the water stops altogether.

The house has a combi boiler fitted already which is in the cupboard in the bathroom and the water tanks have been removed so I don't know why the pressure up stairs is so low and how can I increase the pressure throughout?

I want to know if I get a shower should I get a power shower or a normal one?

Any help would be appreciated thanks