Just to answer my own question:
The top on the miele G 5210 SC dishwasher definitely can be removed and it’s flat when you remove it.
This is what it looks like.
https://i.imgur.com/Q6NgeDt.jpg
(The forum says I’m not allowed to embed the image as I’ve reached my quota)
I was able to fit it easily under a counter which has a 815mm space beneath it.
Looks to be about 5 to 10 mm more clearance.
It is possible, I believe to, raise all the legs from the front after it has been installed although I haven’t tried that and might not.
To remove the top, you open the door and there are two screws above. You remove them and then pull the top towards you and then up.
Here are the instructions:
https://i.imgur.com/iyLc1QL.png