Water Pressure is very low

Can somebody tell me why the water pressure in my bathroom has become very low after removing the pump? I thought the the pressure should revert to what it was before installing the pump but for some reasons it is now very very low. what could be the reason and can something be done about it such as converting the water source from the mains rather than from the tank and/or fitting a pressure valve of 2.5 to 3.0 bar? Please tell me and also recommend a good plumber who can do this work at a reasonable cost.