This app was mentioned in 2 comments, with an average of 1.50 upvotes
Tube Map app has the map layout - plus line status, route planner and rail map. You pay a couple quid to remove the ads but its worth it - ive been using it pretty much daily for what must be close to a decade now.
https://play.google.com/store/apps/details?id=com.mxdata.tube.Market&hl=en_GB This on Android.
Paid like 40p or something. Best carriages and lots more.