A book exchange ?
My readers are so wise. It is actually the village's lending library. A quick Google confirms that several villages in the UK have turned phone boxes to this purpose. This one was in Coed-y-Paen which is very close to Llandegfedd Sailing Club. It was in the news when the library was hit by a gang of book thieves!I guess it was the books that gave it away?
I was going to guess that you are in Brobdingnag.
My guess was a well stocked but very un-private outhouse, but a lending library makes more sense now that you've told us!
Does it smell of wee like every other phone box I have been in?