As @neubery commented, the London bus map is divided into six reasonably sized spider-maps, namely,

1. Central London Bus Map
2. Central London Night Bus Map
3. North-West London Bus Map
4. North-East London Bus Map
5. South-East London Bust Map
6. South-West London Bust Map

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.

As @neubery commented, the London bus map is divided into six reasonably sized spider-maps, namely,

1. Central London Bus Map
2. Central London Night Bus Map
3. North-West London Bus Map
4. North-East London Bus Map
5. South-East London Bust Map
6. South-West London Bust Map

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.