Have added to your list – we felt the A470 around Rhayader has some especially bad lay bys. They are near a large Burger King and a strategic (trunk) route but they have put in proper picnic stopping points on some of the laybys with benches and tourist information signs, almost inviting people to stop off but no toilets or bins provided! As you can imagine they are very unsanitary and alot of snack food litter, not just fast food but like sandwiches and crisps alot