I should have thought you'd be OK with magnolia providing you are willing to give it a very brutal prune every other year. We inheirited one in the middle of our patio and I keep it at around 4m high, but it has a 6m spread and looks fantastic in flower. Very fast growing though - you need to commit to the pruning.
You might like to consider a cherry tree on a dwarf rootstock - these are supposed to reach only 10ft. Again they are fast growing. DO NOT get one without the dwarfing rootstock as they grow to 10m!
Possibly your best alternative is to go for Acers if you have acidic soil. No blossom, but fantastic leaf colour and shape, slow growing and not too large.