Re:
Fork blade light boss bolts are uniquely British and are 5/16" BSC, even if the rest of the bolts on the bike are metric. Hucksters sell plain steel ones for silly rare/vintage money in the cycling section on eBay, but hop over to the motorbike section and you can get stainless ones much cheaper, since they're found on a lot of Triumph motorbikes (I have no idea what they're used for on a Triumph). Metric spanners don't quite fit the head of these bolts - IIRC a 14mm is close, but you're better off digging out your AF sockets.
Of course, nearly all bikes, even new ones, have imperial threads for the BB shell, the fork column (if threaded) and the rear hub (if it's a fixie, singlespeed or has a multi-speed freewheel).