Optimum performance of the upfiring Atmos speakers is a flat ceiling, 8-10 feet in height.  It's still going to sound good, but the Atmos effect will be lessened.

If you want to hear height channel effects from Dolby Atmos content, I would go with another option. The height effects will get lost with the slanted ceiling and skylights.

You might consider getting the Beam (Gen 2). It does a pretty good job with Dolby Atmos audio without using upward firing drivers.

IMO if your space will be similar to the picture the biggest negative are the skylights more so than the ceiling slant. The skylights are where the upward sound would suffer a significant loss as the travel path to the rear would be broken. Think walking and suddenly falling into a hole.

To compensate I'd recommend placing two Sonos One SL’s on either side of the main seating area to create a more substantial rear surround. If you intend to wall mount (TV and sound bar) IMO the Arc will still serve you better in the long run as it is more powerful than the Beam2. If the TV will rest on top of a cabinet; that can also accommodate the Arc, you might consider trying both to see which you prefer. In any event I think Sonos One SL’s as surrounds are a must.

I have a slanted ceiling here but running a different direction. No skylights. Mine is high in the corner behind the TV and low in the corner behind our chairs. Still the Arc ATMOS sounds decent, maybe not as good as with a flat setup.

On the other hand a Beam struggled to fill that room, even with a Sub, the Arc and Sub do just fine there.

Thanks for all the advice. I think I'll still get the Arc and Sub (especially as I've got 30% off via the upgrade programme ). I'll see how it goes before maybe adding the One SL's. Only problem is the wife, she doesn't want the extra rear speakers . I've read the sub is worth adding but how much extra does adding the One SL's give?

Thanks, 

Barm. 

Had the same spouse issue, she hated all the Sonos recommended mounting options. I went with both stacked on a bookcase but rotated 90 degrees. Sounds noticeably worse than the recommended locations but a lot better than no surrounds.

Yes, zero side to side separation and seven feet off the floor.

Look for options the spouse doesn’t object to and get them. Someday you may get asked to make the rears sound better and you can negotiate a better placement.