Westminster Council in collaboration with RingGo have an online app to help motorists find available local parking spaces be they metered or resident parking.
A sample screenshot is shown below, this can be viewed as both a list or local map, outlining available spaces and how busy they are, all in real time.
For more information see https://ringgo.co.uk (we have no affiliation to this external link) or visit the App Store for your mobile device.