Water pressure in showers terrible after new bathroom fitted

most of these showers have filters in them check them first,builder fitted them you should have got a plumber to fit them !,have your water pressure checked,check with shower makers specs for the showers,unfortunately every one now thinks they can do plumbing,so we end up with these problems,he has no control over water pressure,your water boards legal obligation is to supply at a minimum of .7 of a bar,what is your hot water source ?,ie combi boiler,tank and cylinder ?,did builder chose the shower fittings,get a decent qualified plumber to look at it,and check his qualifications, for a real professional plumber,nvq3 with at least four years post training experience,look for a ciphe plumber.this is an unvented cylinder plumber who put it in must be g3 qualified and should have checked there was sufficient water pressure needed to work this system,check your volume and pressure at a time of high usage ie 6am to 9am and 4pm to 8pm best way to check get a plumber to do it with pressure gauge and flow cup,if your megaflow has been installed correctly you will have balanced hot and cold through the control vale on the unit ,this has a filter in it and may be blocked,tell builder it is not fit for purpose and get his plumber back to sort it or you will go to trading standards.

hi full respect to the above reply /answer - a more simple answer would 1 both the showers fitted could be for a higher pressure system being above 1bar to work correctly . or the most common cause if your sure the original pressure were good is the on off gate valve have seized halfway closed or a blockage . I always change any old on off valves with new lever valves to avoid any issues like you have . just ideas and they may or maynot assist you !