I'd take a guess that the owners didn't realise it should be pruned back hard in March or April, they let it get too large and out of hand, and in desperation have cut off all the lower limbs to keep all the growth at the top. You could do that too, but the flowering is usually better if the plant is pruned yearly in the manner I mentioned.

Hmmm that might be it, but the main stem/trunk must be a good 3-4inches thick. A good amount of flowers on it too.

Last year I had to help clear a very overgrown garden - I could see there was a Buddleia in flower, quite near the ground.By the time we'd cleared it, that Buddleia's topgrowth turned out to be about 12 feet away from the root, off a thick trunk about 8 inches across, which had, some time years ago, split near the base of the plant, remained alive, and grew horizontally along the ground. So yes, if its been made into a "tree" for a while, it would certainly have a trunk that thick.

At Hidcote there is a beautiful example of a Buddleja alternifolia grown as a standard rather than as a multi-stmmed bush. The trunk on that must be a good 6 inches round. It is not hard to create a standard like that with any of the Buddleja, it just needs a lot more maintainance.

Maybe it started out as a standard and is just really old, it must be a good 6ft high. It's beautiful.

Well, if you want to create your own, take a good straight cutting of the one you want and root it. then remove any side growths as they appear, but leave the top growing point alone. Remove also any thing which grows from the roots, except your main stem. Keep it growing up wards on that single stem until it reaches the height you need, then pinch out the growing tip to encourage side shoots. Again only allow the top few to grow, any below then should be rubbed out when you see them.