Slings were most definitely used in Wales, as slings might have exceeded the importance of the bow across Iron Age Europe.
https://warwick.ac.uk/fac/cross_fac/iatl/reinvention/archive/volume7issue2/swan/However, I think you’ll run into problems finding Wales specific evidence. There have been a few cases of sling projectiles found, but they generally seem pretty ignored. There are almost no pictures.
(Links may be broken, manually copy entire link)
https://museum.wales/collections/online/object/97d4f024-8dfb-371a-a42d-d1dcb1606c8e/Iron-Age--Roman-stone-sling-shot/
However, I was able to find a few pictures with one definite sling bullet. These possible Roman “ballista balls” are actually the size of large sling stones. How do we know these are sling stones? We don’t, except for one. One of the pictures has a biconical (American football) shape, which is one of the most aerodynamic shapes and a good indicator of a sling stone.
https://museum.wales/collections/online/object/771c7786-68d0-353e-aabc-a44d75e22e91/Roman-stone-ballista-ball/
As of now, this is the best I’ve seen. I don’t know why there isn’t much of any data from Iron Age sites (lack of welsh archaeological interest?). However, we get a lot more evidence during the Roman period. For lack of any better evidence, it’s probably a safe bet that sling usage was fairly similar to all the neighboring regions, probably lasting throughout the medieval period and possibly into fairly recent times.