I stayed at St. Athans with two friends for a single night between arriving in London and leaving for Edinburgh. We booked a three-bed room with en-suite, which was clean and cozy but very small once three people and their luggage had piled in. The staff seemed a little overwhelmed by summer rush and we had to wait past our check-in time so they could finish cleaning our room. We had to run down to the front desk at midnight because they forgot to put towels in our bathroom, and they seemed hesitant to offer them (even though they’re included with an en-suite room).
There is no AC or fan and (as of July 2022) there’s pretty serious construction happening across the street, so it was hot and loud inside the room. At night both the temp and the noise were pretty bearable, but just keep that in mind! The location is lovely.