Before we start checking the water flow, check that the stopcock is fully open. Monitor the water pressure at different times of the day to see if it is affected by levels of demand on the system. There is a simple test to help you.
Check how old your pipework is and contact your water authority to see if there are known leaks or repairs in the area.
If it takes some time for the hot water to come through after turning the shower or tap on, the plumbing may suffer from particularly long pipe runs. This can be improved by installing a shower pump or mains booster pump.