Hello.
By Google Navigate I presume you mean Google Maps?
Google Maps has many uses for a cycle tourist, notably the possibility to save areas off line including places (shops, hotels etc.) that have been saved but as a route planner and a navigator I avoid it.
Google Streetview has uses for checking out road types and surfaces if away from home.
I have no idea if it is linked to Sustrans but in NL, a country with no shortage of bike paths and routes, it made some pretty silly suggestions.
Also, to use it requires a phone and holder which raise their own issues with respect to battery life, weather and damage. Cycle route planning will only work with an internet connection. (You can follow without).
I like to think of bike navigation in two parts:
1. Is the Planning
2. Is the following.
For planning I prefer cycle.travel.
For following I use a gps device.
If you do a search you will find many threads discussing the merits of one planning app over another and the different units.
You could do worse than download Osmand if you are new to bike navigation. It's a bit clunky but it works (offline too), has some pretty impressive features and is a great introduction to gps for bikes.
P.S. Are sustrans routes not signposted?