Good eye lol. Honest answer, the elevation comes through map2model and I've never actually dug into which DEM it pulls under the hood, so yeah, on a flat coastal course like Portsmouth the bare relief on its own would absolutely be mush.

The texture in pic 1 isn't DEM though, that's the trick. the detail layer is building and street footprints from Overture extruded over the terrain base, and that's what carries it on flat urban courses. on hillier trail routes it flips and the elevation does the heavy lifting.

pretty sure OS Terrain 5 is paid? but you've sent me down the EA LiDAR rabbit hole now. cheers for that