I interpret this as being able to put in a 18" deep piece (in the Y direction, away from the user), so yes, you’d be able to put a piece of material in which you could cut the majority of your 300mm circle.
So yes, you wouldn’t be able to cut the whole perimeter of the circle which it sounds like it doesn’t matter to you.
Furthermore, based upon what I’m reading above, it looks like your circle would look like this:
Where the distance the work area is from the front of the bed area determined by [quote=“dan, post:36, topic:3587”]
The board would be much closer to the front than the back - 1-2" from the edge (I don’t know offhand).
I think that should answer your question?
In addition, if the center of your 300mm had a hole in it, you could rough saw your workpiece and use a jig with a dowel in the middle so that you place your workpiece on the jig, centered on the dowel, then cut as much of the circle you can, stop, rotate your workpiece around the dowel, and finish the cut.