I want two trees at the front of the house to provide dappled shade in the summer. The soil here is very chalky with a good dose of stones and hardcore. There’s no support for the trees. The site faces West, but late afternoon light is blocked by other houses.
Good blossom is a must and the foliage should be interesting. I’m thinking along the lines of a cherry tree and something else.
Conifers are out.
What do you recommend?
There are some really nice ornamental crab apples; prety much any choice of blossom colour from snowy white through marshmallowy pink/white all the way out to wine red. Foliage is pretty ordinary on those.
Amelanchier of some kind might fit the bill; lots of white blossom, nice smallish deep green leaves, berries in June and great autumn colour (although better on acid soil, but they will tolerate chalk)
Or how about a nice laburnum?
You’ll have to be more specific about your geographic location than “Demographic Anomaly” if you want a usable answer. 
Trees are normally recommended based upon Tree Hardiness Zones, (as classified by USDA, although there are variations on these). Generally speaking, these zones refer to the lowest average temperature that might be anticipated throughout the year. If you give a general idea as to your location, that would locate you in a specific zone, and certain trees would be ruled in or out.
Zones are the first and most important factor in determining suitability, but it goes on from there. Other considerations are wind, sun, salt/ocean exposure, rainfall/moisture, etc.
The more information you can give, the better people can help. I can probably give some good advice for the US from the northeast through some of the midwest, possibly the Pacific northwest. If we are south of Virginia or in California I will sidestep this one.
Some further questions:
-How tall do you want the tree to get? How far up your house do you need shade?
-How quickly do you need it to get to this height? Faster growing trees tend to be weaker and messier (broken branches). Usually shorter lived, as well. Folks who are looking for a quick landscape and don’t intend to be there long tend to go with the fast growers.
-Do you have an irrigation system, or is water an issue?
*Laburnum shirlii * is nice, I think. Does well in Milwaukee. 
I agree about flowering crabs. Wyman tends to favor them over flowering cherries. As with almost all fruit trees, vigilance must be maintained for pests, although there are some resistant species. But, let’s get a little more information before making that decision.
I’m in Luton, U.K. Water is not an issue, nor is fast growth
I’ll step aside from making a definitive recommendation on this one. I’m just not familiar enough with the climate or growing conditions to comment with any authority.
That said, I have no problem pulling a suggestion from the lower parts of my anatomy.
Nurseries in the UK carry a cherry that is difficult to find commercially in the US. It is the Paperbark Cherry (Prunus serrula). It grows to 15’-20’ in height (is that tall enough?). It has the reddest and shiniest bark of any cherry, which gives it landscape interest all year long. It does have flowers, although they are not as significant as other flowering cherries. Fall color is yellow.
I have a tree in my front yard that may grow where you are. It is the Dove Tree (Davidia involucrata). This one is a labor of love; it can take ten years or more for it to flower. The leaves are very coarse in texture, and the branching is picturesque. I thought the texture would be unattractive, but it is actually quite interesting. If you have the time to wait for this one, and if it will grow where you are, it will be a very distinctive tree.
Sorry I couldn’t be more help. I’m sure that the UK Dopers are much more familiar with local flora.