When considering your shower first think about pressure. If you home is equipped with a combination boiler or unvented system there will not be a problem. If your home has a gravity system than a pump will need to be installed. Check with your installer. Most shower valves are thermostatic which will ensure the temperature remains consistent even with a combination boiler. If you choose a manual valve remember that when another tap is turned on in the house the shower may rapidly get very hot or cold. Not a pleasant experience! In Sussex we have very hard water but the modern thermostats are very resilient with guarantees of up to ten years. When using a built in valve ensure the wall has a cavity deep enough and your installer can fit it without problems.