In article , Janet Tweedy
writes

Well to me Cornus is small because if you want the red stems you must prune at
least a third of them out every year and even the larger cornuses (ii?) aren't
going to shade our living room very much I've got about four dogwoods but are
you thinking of a specific one?

Cornuses aren't just about red stems! There are many species, grown for
showy white bracts, berries, or elegance of form. Try for a start C.
kousa, nuttali, mas

I've only just come into this thread, but picked up you want a 20ft tree
and would prefer a british native for the wildlife - have you considered
whitebeam? Opening leaves show their white undersides and look like huge
white tulips, then you have white flowers, and berries in the autumn.
--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/edward/index.htm