If you have a hot water tank ( gravity fed ) The hot water pressure is a result of the height of the water feed tank above the tap or shower head. 22 mm pipe is normally used from the top of water tank to the bath taps and a short length of 15 mm pipe to other taps. As Peter said a shower pump that feeds both hot and cold will help also a pressure reducing governor on the cold supply may help by balancing the different pressure between hot and cold. The simplest solution is to raise the height of the small ( feed and expansion ) tank the feeds into the bottom of the hot water cylinder by putting it into the loft. If you look online for hot water gravity fed systems.