May 2017 edited November 2018
I asked this very question at the DfT’s Traffic Signs Technical Working Party in June 2016. The answer I got (as it appears in the official minutes) was: “WD [DfT] replied that it could possibly be installed using the Planning Regulations, whilst MY [Highways England] thought that the Environment Agency had their own powers to install these”.
More recently, a draft of the new Traffic Signs Manual Chapter 4 has been given a limited circulation for ‘peer review’. Whilst discussing fords, it makes no mention of depth gauges. The IHE made strong representations that they should be included, as their omission from Chapter 4 implies that DfT thinks these devices are no longer necessary and that authorities may remove them. The IHE considers that would be dangerous and could lead to lives being lost.