The funeral is going to be limited to 30 people. The royal family alone has enough members to fill that out. I doubt heads of state from other countries will be invited.
Lemme see: 4 children, 8 grandchildren, and 10 great-grandchildren. That only adds up to 22, and the little kids (the great-grandchildren) probably won't attend, anyway. But all of the children and grandchildren are either married or have been married, and it's entirely possible that exes such as Fergie might attend. So 4 becomes 8, and 8 becomes 16 -- that's potentially 24, right there.
That would leave only six slots, and that's probably not even enough to allow the heads of state of all commonwealth nations to attend. So my guess is that the funeral will be limited to family.