The best listings of nationalised brickworks for 1947 are in:
Colliery Yearbook & Coal Trades Directory, 1947 ed.
Guide to the Coalfields, 1947 ed.
Directory of Quarries, Clay Pits & Brickworks (the title kept changing the order of the words between editions); unsure of which edition was closest to 1947.
In south Wales many nationalised brickworks were placed within the wholly owned NCB subsidiary, the British Anthracite Company, which also operated the some of the railway wagon works that were taken over at nationaluisation.
If you combine the information in these three directories you should get close to the complete list.
The National Brick Collection of the National Museum of Wales holds over 600 different Welsh-made bricks, tiles, pipes and refractories, most of them from south Wales. A small selection, plus a Bradely & Craven brick making machine from Emlyn Colliery Brickworks, are on display in the National Waterfront Museum at Swansea. The reserve store that the remainder are housed in is closed to public access for around a year due to building work.
From recollection, only three Welsh brickworks actually put "NCB" on their products: Onllwyn, Tredegar and a third that I cannot at present remember.
Brickworks that were owned by British Anthracite that I can recall include Cynchordy, Goodwick, Trimsaran though some were absorbed long after 1947, having remained independent for some years. So although ultimately owned by the NCB, they would not appear on the list of brickworks taken over in 1947.