I emailed James about this problem. The plastic back boxes work ok, although the Loxone switch is slightly smaller so not a completely neat finish.
UK metal back boxes are an absolute nightmare with the Loxone switch. The back plate just wouldn't sit correctly. In the end I sliced the face of a plastic box, superglued that to the back plate and then screwed it in place. I was finally able to click the switch into place.
Anyway, use a EU standard circular back box. Easier to fit, one securing screw in the centre of the box to fit in the wall and you just turn it as you wish so the switch sits plumb.
The back box is slightly smaller than the switch so you get a really neat finish.
Use a diamond core drill bit to cut the hole for the box, if in a brick wall. Much better than faffing around with a bolster or SDS drill, neater as well.