I think there might be a feasible fix for your London area (only terrain).
Includes a new "terrain.cfg" file.

File name via "avsim" - "fsx_modified_terrain_cfg.zip"

Check it out, it might solve some issues with terrain, not including though bridges.  

BTW it's also, currently being uploaded to SimV.

Nice pick up Skynoz

Well, congrats to Rhumbaflappy and Luis Féliz-Tirado for making a first stab at solving the problem. The results are a big improvement but really this is unfortunately a partial and at best a temporary solution.

They used all the horrendous London terrain errors as their examples and set out to try and do something about them. What they have managed to do really is just convert all the rocky lumps and bumps into - water lumps and bumps.

So all of the mesh errors that I've checked are still there but they have now been made to look like water. This means that you have waterfalls entering the Thames, large deep creases in places in the water surface and, at the Dartford Bridge, what looks like small tidal waves.

At the Humber Bridge there is a large water crease into which the AI boats disappear and suddenly shoot out the other side.

But anyway, it is an improvement and worth installing as it's only a very small replacement for the default terrain.cfg file