Recently I solved the problem regarding the mismatch between British National Grid CRS and WGS 84 in the query below:
I downloaded the .gsb file and copied it into the relevant folder like shown in the instructions.
However now I see, that this .gsb file (with custom CRS) is valid for one layer only. I can't see it for other Ordnance Survey Maps...
You can see the layer above with "Unknown CRS" (located in unknown Coordinates System section) which comes from the copied .gsb file unlike another layer below, which doesn't have this layer in the search box at all.