Was there any discussion about this? I can imagine the Welsh in particular not being happy. It's officially a bilingual country, just like Switzerland or Belgium (they have 4 and 3 official languages respectively).

If you can convince the community (not just me) that your coastline follows MHWS more accurately than the OS, then OK. I doubt you can because you admit to basing your work on a single snapshot photo - with all its inaccuracies and other unknowns. Otherwise please consider reverting your edits to the published OS vector data, or take this discussion to the mailing list.
Note it's not about being outdated, it's about the mathematical principle that you cannot derive a long-term (1 year+) average from any single photo, or survey for that matter. Please see https://www.pla.co.uk/Safety/Tides-Definitions-and-Notes for information about the algorithms - can you follow that better than the OS?

It's really not a good idea to infer coastlines from aerial imagery. The coastline is defined as mean high water springs, which is taken as a long-term average and no snapshot can be good enough, even if you could detect where the water stops and the state of the tide at the instant the photo was taken. There was a recent discussion on this subject on the Talk-GB mailing list: https://lists.openstreetmap.org/pipermail/talk-gb/2023-October/030770.html

When you say "OS Maps" do you mean the published vector data in (e.g.) Boundary-Line, of something else? As admin boundaries are aligned to Mean Low Water Springs (in Scotland) the same comment about aerial photos applies here as well.

Hi! Can you tell us what your source is for the admin boundary (LWM) and coastline (HWM) data? It doesn't seem to correspond to OS data and the combination has anomalies (coastline beyond admin boundary for example)

How do I view a map with the label language governed by my browser preferences? AFAIK on osm.org all the tiles are rendered server-side and the language choice is fixed.

Sorry, I mean the admin boundaries are aligned to MLW of course!

I would suggest to just bear in mind that UK admin boundaries are in general aligned to MHW whereas the coastline is MHW. If you find yourself wanting to set the coastline seaward of an admin boundary, one of them is wrong. But it often happens that MHW and MLW lines legitimately coincide (when seen from above) in areas of steep cliffs or man-made things like harbours and breakwaters. Watch out with piers if they are built out over the sea, as the end of the pier can be not only beyond the coastline but also beyond MLW, although they will most likely be included in the admin area.

Regarding "undoing someone's work..." that is exactly what you are doing...

Just realigning the data to its tagged source. Feel free to tag your changes as "personal guess based on a single aerial photo" and allow others (including me) to decide which is the more reliable source. Of course you are free to survey the area yourself over a period of months and account for tidal variations due to atmospheric pressure, seiche waves and other phenomena...

Imagery will not help! You cannot derive MHW(S)/MLW(S) from any picture, you need a professional survey over a long period of time.
Another advantage to using the OS data is consistency between low and high water lines. You will never find high water lines seaward of low water in the OS data. It can be co-linear (vertical walls and cliffs for example). OSM is full of examples of these logical errors caused by coastline and admin boundaries coming from different sources, and the OS is the best (public) source you will ever get.

Aerial imagery is no good for that as you have no idea of the state of the tide when the photo was taken, and you can't really tell if rocks are covered with water or not. In the UK the best data for HWM and LWM for us is from the Ordnance Survey in the Boundary-Line data set which contains HWM data and government boundaries. The coastal data is reviewed accurately every few years. This is what I use when updating coastlines and coastal boundaries.

Hi... Where did you get your coastline data from?

Shame you didn't fix the actual problem (tag centre should be admin_centre) instead of adressing the symptom

Please note that admin boundaries are not based on logic or common sense, but on legal documents. The Glynneath community boundary is what it is.

Cancel that last sentence - I see that the NI councils have been set to admin_level=7, undocumented in the wiki. So much for consistency!

What is their (small) administrative function? Are they different to the Lieutenancy areas in GB, which are basically the "historic counties" and tagged as such? Not every boundary defined in law is an administrative boundary. In OSM there is usually a check that it has some kind of democratic constitution ("has an elected council"). If you want to diverge from the established tagging scheme in this way it might be better to ask the community first.
Having two sets of admin boundaries at admin_level=6 is also asking for confusion.

These counties are no longer used for administrative purposes and you should not be retagging them to boundary=administrative! Please revert them to boundary=historic.

Please revert this. The underlying admin_level=6 areas should not be boundary=administrative - they were recently changed in error. Once again you have made an "error message" go away without actually fixing the problem correctly.... Please be more careful. This lowers the data quality, not improves.

please don't use boundary=fence - use barrier=fence instead

This multipolygon is topologically incorrect - one outer ring is partially within the other outer ring. osm.org/relation/16253713