I'm sorry to have to report this. There are so many things to like about FSX that it's a pity it's being let down for us in the UK by these glaring mesh problems.
The Humber Bridge has the same problem as the Dartford Bridge (worse actually) and in fact there's a serious mesh error that extends along quite a length of the river upstream from the bridge itself.
I just checked out the Severn bridges and they were both stunning and the road system in the area (M4, A38 etc) was great with all its moving traffic.
However, flying upstream from Avonmouth to Bristol there are a whole series of serious LWM/mesh errors that badly affect the appearance of the area giving blockages in the river and differences in levels of both adjoining terrain and water.
There does seem to be a problem with the mesh system in the UK which in fairness to UK purchasers needs to be resolved very speeedily in my opinion.