Poor water pressure can be down to many things, but the main cause is clogged pipes. If you have already replaced the pipes, try the obvious tests first. Make sure the systems shutoff valves are full open. Yes a basic requirement, but you would be surprised by how many have been turned off in the past, but not been opened properly following any works carried out.
Other reasons for low pressure can be down to being on a well system. The pump can only do so much, so you need a pressure tank to boost the pressure to the house. Leading from your boiler their should be a pressure gauge and recommended pressure is 1.5-2 bar to keep that consistent pressure to your taps.
If the pressure reading is adequate, you may have obstructions or some pipes that are too small. Call in a licensed plumber to evaluate your system.