Top tube downslope angle

we are having 20mm of spacers - he does not want to extend the head tube relative to the top tube as it will compromise front-end stiffness (particularly as we are not going down the integrated head tube route).

well 20mm isn't that much.
i don't agree about an extended head tube compromising stiffness though, quite the opposite.
my cynical nature suggests the builder doesn't want to source a longer tube than the ones he has in stock :-)