I have rather largeish sussexes (by north american standards anyways, they would not hold a candle to anything in the UK!).
They are just, you know, chicken-sized
I would not give them less than 18" over the roost if you can possibly avoid it, especially if you have a rooster (they need to stretch *up* to crow). They can squish into just 12" or so of headroom but not enjoyably and if you live somewhere that gets cold winters you may have condensate/frostbite problems with their heads so close to the ceiling/wall.
In an A-frame ark you really don't have a lot of choice, of course... I suppose it's too late to talk you out of that design as you've already made it, but it really doesn't provide very good quality living accommodations, mysteriously-popular though it is in British backyards.
If it were me, I would err on the side of putting the roost lower (even if barely above the bedding to give them more headroom, rather than raising the roost with the result that they have *both* not much headroom *and* the roost is constantly in their way when they are trying to walk around in there. With the low roost you may have some trouble iwth them sleeping in the nestboxes instead of on the roost but frankly that is just an unavoidable hazard of most A-frame type designs and you should probably just accept that fact and not owrry about it when deciding how to locate the roost.
Good luck, have fun, it's a really nice breed,
Pat