It wants to avoid the short stretch of road by Teddington Locks, so it sends you on a 3 mile detour.
OSM's routing layer used to be missing loads of the interconnections, so it wouldn't send you down the obvious route because it though the main road was a cul-de-sac and wasn't connected to the next bit of road. It's got a lot better recently though.
OSM maps seemed to contribute to more lockups/crashes on the 810 so I use Garmin maps. I will try Garmin maps on the Edge 1000 to see if they also give shit routing options.
Here's a rather extreme version for Google on walking mode:-
https://goo.gl/maps/xLCswgb9H5H2
It wants to avoid the short stretch of road by Teddington Locks, so it sends you on a 3 mile detour.
OSM's routing layer used to be missing loads of the interconnections, so it wouldn't send you down the obvious route because it though the main road was a cul-de-sac and wasn't connected to the next bit of road. It's got a lot better recently though.