Maybe a compromise is possible, I nearly always use 5lb line for my general fishing but it is fair to say I don't have Severn barbel or any barbel come to that but I have landed some hefty fish on it on occasions. Maybe to heavy a line for roach for some dedicated roach anglers and I may have less roach bites but I still catch them and I hate losing fish to a snapped line so, I feel confident-ish if I hook into a bigger fish like a chub or mullet I can land them; in fact it is very rare I get a snapped line and it is more likely to come away if snagged but not always if it is hard snagged. I don't like those ultra fine lines, I am always afraid of breaking them one way or the other. A few less fish maybe but feeling confident about landing them makes up for it personally.
Pushing a piece of crust onto a 14-10 hook with a shot about 2 ins from the hook so the crust just sits up off the bottom I find quite deadly for roach and the smaller roach tend to leave it alone a bit. If they nibble at it a big roach might still come and take it. When you think it may have been nibbled away a bit strike it off, reel in and re-bait, this sets up a trickle of bait and I often find after an hour or so of doing this the fish start to find it. A easier tactic if your float fishing though, maybe a bit tiresome if your feeder fishing. And if your catching them on pellet, they may be better, I would try them more but eels are a problem in many of the river places I fish,
I only use a standard 12ft match rod for fishing for roach which I find does for most fish but again, not for barbel but I could still land one if that was the case and a 5lb line is less likely to snap. Not so good for feeder fishing though but I dont like feeder fishing much, all that weight and sploshing about. I used to do it for barbel fishing on the Avon once but swapped it for just light legering or even free lining a big bait; much more satisfying in my opinion and on days it fished better than a feeder even though they are often deadly and fish get used to them but they snag up and using rods designed to use them, I prefer a rod designed for the fish to be honest.
I find corn selective for roach Phillip in that the smaller fish tend to leave it alone or at least do not harry it as much as bread. My standard way is throw a few small handfuls of mashed up bread in and a small handful of corn and alternate between the two on the hook. If the bread is getting harried to much by small fish I leave the corn on. I find it gives less bites but the fish tend to be bigger.
However, having said all that it is all how and what we find in very varied scenarios and what you like/find/suits best.
