Roots out growing pots. HELP!!!

I have 8 UK cheese plants in a DR100 grow tent
1x 600MH veg
1x 600HPS flower
BioBiz All-Mix soil
BioBiz nutes feeding very lighty
I had to transplant them from 10ltr pots into 18ltrs as they were becoming root bound. I have returned 3 days later to find the roots have out grown there new 18ltr pots and coming out the bottom. Im onto the 3rd week of flowering now and wouldnt want anything to put this lovely crop in danger

Any Help appreciated.

I'd just laeve them be myself. I have never looked into the science of plants being rootbound, but i don't really buy anything from it other than it meaning the pants have filled the pot, i don't beleive there are negative consequences other than the plant won't grow as could have been potentially possbile. Only difference i can see si that a pot with lots of roots in it is going to drink the volume of water possible faster than a pot with less roots in it.