I wonder what is going to come of this since ROH always runs "Supercard of Honor" that weekend. If NJPW did a joint weekend where ROH had a show one night and they had a show the next I could see how that would be beneficial, but trying to take ROH's spot and/or compete with them doesn't make much sense to me.
If it were up to me, I'd reach out to ROH and Rev Pro about doing a fan-festival event in London that weekend instead, and try to entice all the normal fly-overs from Europe to stay somewhat local and come to a big ass joint event closer.