My experience with the Crystal Palace in MK is that they do not open any earlier on EMH days. This means that early ADRs are counter productive on these days since you sit and eat while the park is open during the least crowded hour of the day. I had to cancel my ADRs when they announced the MK was having EMH every day at Christmas for this reason.
We have gone to CP twice for an 8:05 ADR when the park opened at 9:00 and loved being the first ones to ride whichever ride we chose that morning.
Good Luck with your planning.