I'm not sure JP223 actually "fires" the submunitions though. Could be that they're just gravity-dropped out of the opened chutes of the dispenser?
(with the two settable dispension patterns achieved by altering the "opening pattern")
MW-1 uses pyrotechnic charges to actually "launch" the submunitions to the sides, grenade-launcher style.
The sideways launching of course allows the aircraft to fly even lower in an attack run.
YouTube - Tornado GR1/IDS tribute
JP223 at 3:40, MW-1 at 3:46. Note that that's in slow motion.
We used the regular Tornado and F-16 overflights over our base for AA MG training with blank ammo
Well, not at treetop level, but at around 200 feet above the base - since we were on a mountain, and they were regularly coming in at about 2000 feet above the plains below.