Low mains water pressure and flow is quite a common problem in the UK especially more so in London. The reason for this being that as properties are smaller, people are more likely to install combination boilers. A 24kw combi usually has a delivery rate of 9 liters per minute, whereas a 37kw combi has an approx rate of 15 liters per minute.
Most water companies are required to give minimum 9 liters a minute at 1 bar to the property. So as you can see the bigger combi boilers will require increased flow rates. Should you have multiple bathrooms and require an unvented hot water system, your flow rate will need to be minimum 20 liters per minute if not more.
We at MyBoiler.com Ltd are able to arrange for upgrades to you mains water pipe from your meter / external stopcock to your home, so why not contact us to arrange for an assessment/ quotation. Contact Us
Additional Resources:
Understanding Water Pressure: http://www.thameswater.co.uk/help-and-advice/16159.htm
Understanding Water Flow: http://www.thameswater.co.uk/help-and-advice/16840.htm