Hi Malcolm.
Colin hs the right route if you like motorways, but you could run into traffic from j11 m6 to j8 and again on m5 from m6 to j4 m5 and at j6 and j7 worcester. There are roadworks at the island below j24 M4 Newport which seem to be taking forever to complete.
IF you don't mind A roads, the A49 from Warrington to Leominster and then A438 to Brecon, then the A40 to Carmathen. It is all good to reasonable quality A roads,and reasonably direct. It's a long treck which ever way you do it, but on the A road routes offer several rest points. At my town Leominster for instance, you can pick up tastey fish and chips from the chippy at the main traffic lights which has the old bus station car park behind it for easy parking. you would also go past our Morrissons store which again offers food and fuel with easy parking. After then the next handy stop over is Carmathen Morrissons. There are neumerous layby's along the way.
Atb Steve L.