score:4
As @neubery commented, the London bus map is divided into six reasonably sized spider-maps, namely,
You could simply download the relevant PDFs on your phone, zoom in and plan your journey yourself. I use maps 1 and 2 myself and it's very well drawn! Although it might be a tad difficult to use on a mobile screen, these six PDFs do seem like a simple solution especially since you can search for text on them.