Bill - helicoils are wonderful things - when used correctly. I usually loctite them in to mitigate the risk they'll pick up on the bolt. As for getting your helicoil out, best bet is to unwind it (from the top, otherwise it simply tightens) Try a LH drill just under the tapping size for the helicoil thread and hope it bites at the top. If done slowly, it should unwind. Best done in a mill or at least a pillar drill to ensure it's perpindicular. Then, as Slymo says, drill and tap oversize and fit a bush. That depends, of course, if there's enough 'meat' to do that.
Or, whilst it is non-standard, the fact 8mm is bigger than 5/16 and 10mm bigger than 3/8 might get you out of a fix (provided you don't mind going to metric bolts). And there are better quality bolts in metric than you usually get in Bscy - when you can find them (OK, so I actually hate metric with all its different pitches - cant beat BSW / BSF)