If you cut the head of the bolt off on the outside of the car, you can get the rest thru this hole you made on the inside, or if your not bothered about it - leave it in the sill to rattle around and annoy you.
I thought about cutting the outer sill to get to mine, but was concerned about the weakness it might cause and the fact that you will be able to see that its been cut after its been welded back up. Doing it from the inside takes about 5 mins, its easy.
The pic below shows were you need to cut, but as you can see, this one has had the rear pockets removed, but its still the same idea.
You want to cut the top bit, not the side.