Power shower, hot water tank or combi boiler?

By installing a combi boiler you may increase the flow, however you cannot fit a power sower to a combi system. It sound like you just have low pressure, therefore I would suggest either fitting a power shower on your existing system (the cheapest option) or fitting a pressure flow main system to the entire flat. The 3rd option is to investigate if the main in is sufficient if not have it replaced. Hope this helps.

I assume you have a gravity system comprising a cold water tank serving toilets, baths and whb`s plus a hot water cylinder to store heated water ,perhaps also containing an electric immersion heater. You could fit a pump to the system to give you pumped water to all fittings ( except WC) or to the shower only.The way this is plumbed is important ; you will probably need a "Surrey valve" fitted to your hot water cylinder to prevent air ingress and since these pumps are triggered by a water demand , you need to ensure that your WC is not on the same circuit otherwise the pump will start operating when the WC is flushed. The cold supply to the pump should be taken directly off the cold water tank ie not branched off any other pipe. These pumps can be quite costly and the price seems to rise by about £100 per "bar" of pressure sought. The pumps make a slight noise. I had one fitted to an old gravity system 2 years ago , worked well, great shower with a 300bar pump. I`m selling the pump now though as we are gutting the house completely and installing a new water and heating system.