The recommended drill size for any pen kit is always larger that the tube itself, because you have to leave room for the glue. If the drill size is 11/32, which is 8.73mm in new money, it's likely that the tube itself is about the 8.5mm mark. Even for a tube to push in without any glue, it needs to be fractionally smaller than the hole, so your question about an 8mm drill definitely wouldn't work, nether would 8.5, as the tube still wouldn't push in, but 8.7 probably would work.
Most retailers sell the correct drill bits for their kits, or as has already been mentioned, you can get them from sources like Tracy Tools or Uk Drills
