In reply to comedy_phil:
I don't know Forest Rock at all, but the description of the bolts on the Leicestershire climbs website, together with photo #22 in the Homepage slideshow, very much suggests an actual bolt-head rather than a headless bolt shaft. If this is the case then a keyhole hanger is probably your best bet (as used in said photo), being more secure and generally stronger than a wired nut or wire rivet hanger - especially, as you say, on steep rock. I think that in this instance Doubloons would be a complete waste of time (and money); they're aid-climbing tools designed to facilitate use of barely protruding headless studs/rivets on which pretty well nothing else will stay put. As these studs/rivets would probably be 1/4" or smaller - ie up to about 6.5mm, with modern bolts being generally 10mm or 12mm - I would imagine that they would be way too small as well.
Incidentally, Fish Heads are Fish's copperheads and nothing to do with all this; the picture on his website of a cable loop cinched under a bolt-head is of a rivet hanger.
It might be worth checking the exact size of the bolts before ordering anything made in imperial sizes; I've no idea whether a keyhole hanger designed to fit bolts up to 3/8" - just under 9.5mm - would actually have enough tolerance to fit a 10mm bolt.