hi tom you need a pressure gauge that fits on washing machine outlet screwfix around Â£40 i think check pressure at 3/4 times of day this will give an average pressure and using a timer get the flow by timing say 30 seconds into a bucket count litres in bucket multiply by two = litres per minute or 15 seconds multiply this by 4 = litres per minute using this information you can select shower with confidence all the best john
To check flow rate you need a wier gauge. You can get an idea of flow rate with a 2 litre measuring jug and a watch but not very accurate. For pressure you need a pressure gauge. Flow rate is more important than pressure. Check with the shower manufacturer.