your shower is probaly connected to the cold water header tank in attic instead of mains supply that supplys the attic ,small easy job to do .1 X15mm t connection ,small length of 15mm pipe and 1 x15mm elbow joint is all you need .i had the same problem myself.the pressure to your shower is vastly improved with no flutuation of temperture
paddy derry city ireland, August 2010