I don't like the bed being unsupported over a 3m length. You can at the very least mount a piece of steel box section vertically at the back and the front, but the front one would have to be removed when you make the axis longer. A third piece of 3m long box section in the middle wouldn't go amiss either. Also I would make the bed bolt on to the 4 verticals on the other side, again to make it easier to extend the bed in the future. Currently the frame can 'skew' quite easily, since there are no triangles - so add some pieces to make triangles! You can do this in all 3 planes which is a good way of using up the left over box section since no doubt you'll be ordering in 7.5m lengths. Incidentally how much did Adey Steel quote you for the box section, presumably 60x60x3mm? Their pricing seems to be a bit random...

The way you have mounted the ballscrews is good since it allows adjustment of the height via moving the 4 pieces of box section, which is nice and strong. You can make the end mounts fasten to the box section via holes slotted horizontally to get adjustment in the other axis, which will help when aligning the ballscrews.