This is an email response from Kate at Ashford:
To fold your loom with a warp on it simply do the following:
Loosen the tension off- I prefer to loosen from the front roller. You need to loosen it off about 15cm [6in].
Loosen the knobs on each side of the castle, you only need to loosen them a couple of turns, don't completely unscrew. Standing in front if the loom with one hand on each knob lift the castle up and then slide back. There a is safety feature, so it doesn't slam down and catch your fingers. When you reach the safety stop lift up again and slide all the way down. Once fully folded, tighten the knobs on each side.
I also place a piece of thick paper or card between the levers and the warp to avoid any threads being stretched by the levers pushing down on them.