Download the PDF Tube and London Connections maps from the TFL website and save them to your phone. You'll be wanting to refer to them underground.
Above ground, Google Maps works well.
Several of the major museums, such as the British Museum, have their own apps with maps of the interior.
Other than that, I suggest consulting web search and websites rather than using apps; the information is more specific and up to date.
I assume you'll have a generous data allowance on your device; if not, get a local SIM.
United Kingdom | Prepaid Data SIM Card Wiki | FANDOM powered by Wikia has a comprehensive list and the topic has been discussed in several threads in this forum.