Hi Alan
For fishing the wye (roughly similar i believe to the severn, barbel and chub + boulders/ snags) i use 12lb daiwa sensor as mainline. Its strong and abrasion resistant - its also relatively cheap so I can replace the top 100 yards on a reel every season.
For hooklengths i always use 10lb breaking strain, but a couple of different ones dependent on the conditions. If the water is high and coloured, I use maxima chameleon, which is pretty tough. In "normal" conditions i use a short length of drennan sink braid (say 6 inches or so) to a longer section of the mainline (where the feeder is stopped by a line stop). And if i'm at all concerned that the clarity of the water might be making the fish a bit spooky, then i might try a hooklength of drennan double strength (for it's low diameter) To be honest, that last option is my least preferred, on the basis that it's probably the least robust.