The British book is talking about a place where the temperatures struggle to get past 85F in the summer, therefo0re the admonition to plant it where it can bask in the reflected heat. Most places in the British Isles don't go too far below freezing, so winter survival is not a worry. But if you want blooms, you gotta have heat. (They also have Black Walnut trees that grow fine but never produce walnuts, smae reason, no heat.) Those trees like heat, and 85F is not heat for a M. grandiflora.

So far as the height, I planted a twig 15 yrs. ago and it is about 20ft. tall now, about 8 ft. diameter circle of leaves. This was the 2nd year for bloom. In this climate, they can grow with several leaders. Keeping them with several leaders can make them grow slower and probably will work to reduce overall height. I would guess that you have 20 years before the tree reaches your house. Another thing, in a severe winter, your tree may defoliate. Not to worry, it will come back.

If you are into Magnolias, there are several deciduous Magnolias native to PA. The leaves tend to be larger than grandiflora's, and they are not carboard-y like grandiflora. M. macrophylla will grow nicely here, acuminata and virginiana too, as well as most of the Asian ones.

M. grandiflora will grow quite large up here. I'm in southern Chester county and a bit warmer than your area, but some within a mile or so of me are easily 50-60 feet tall. Once they get over about 2 inches in diameter, it's hard to kill them with the winters we get, although this area lost many in the bad winters in 78 and 86.

George