If you have some kind of smart device and a service agreement that allows you to get data in the UK, then an app may be a big help.
TfL (Transport for London) do their own. Here’s a link: https://tfl.gov.uk/maps_/tfl-go
We don’t use it. We do use CityMapper. Now, this is not a plug for this app, and we could happily tell you the very many things wrong with it, especially the advertising, but we like that it works in different cities in different countries. They have a site: https://citymapper.com/?lang=en
One thing we would say is that free wifi is not the easiest to find in London. Some stores do it the usuals like Starbucks can do too. It’s on some train lines, but few, and some hotels. Tube stations have wifi but not for everyone.
Also, being surrounded by so many buildings can make getting a mobile signal tricky, so this is not a guaranteed method. In our experience, the South Bank is particularly bad.
Other apps must be available. If you find something that works well, do let us know.