Thanks @julia.shestakova
ETRS89 is fine, but DJI drones expects to see ellipsoidal heights.
I think in my case, I wil be using OSGB36 + ODN, which is a Geoid height so elevation value is going to confuse the M300 and altitudes will be off.
I found this really useful thread by @cameron.baker which I think is the solution:
