I'll toss these articles for you two to peruse - I don't personally have the justification to purchase the access for the research article at the moment, but this is extremely compelling evidence that bearded dragons do not absorb water through their cloaca. While people should give beardies baths to give them an opportunity to drink, I personally think it's somewhat intellectual dishonesty propagate the myth that they drink through their butts as the reason for the baths.

This is the original post I found when researching, that started my inquiry

Moisture harvesting and water transport through specialized micro-structures on the integument of lizards
Hydration Therapy This paper is particularly interesting:

At times various individuals were seen to lap water from stony surfaces, or “rain harvest”, lick leaves or evenattempt to extract moisture from wet sand. Curiously, not once did any of them approach a larger pool (6” x 18”) of water that was reconstituted each time the sprinklers were turned on and remained for only 10-15 minutes after they were shut off. However, one particular female P. cornutum when placed in the pool stood up to her knees and drank some water before scampering off.

This is a study of horned toads, specifically, but implies strongly that arid/desert reptiles do not go for pools because they are not frequently available, nor last long enough in nature to be a worthwhile source of water. When placed in water, however, they take the opportunity to drink rather than sit in it.

Osmoregulation is Amphibians and Reptiles I believe this paper is going to be the best resource. Unfortunately, it's also $32. I just can't justify the cost right now, but plan to read it myself later on down the line.

And finally, a quote from the OP of the UK thread later on:

I would like to give a bit of an update too. I contacted a few authors of the studies Ive read lately regarding this subject (Dr Wade Sherbrooke, Dr Glenn Tattersall, Dr Doug Mader, Dr Paul M Gibbons). The last two are very well known vets (Im sure many of you will recognize the names) who teach other vets and write textbooks and other vet manuals for reptile treatments. They universally agreed that lizards do not exhibit any cloacal drinking, as they called it, and that this was considered common knowledge in the scientific field. One of them actually seemed to find it quaint that I was bothering to ask still. I felt a little foolish for not being more up on current research in the end.