Buddleia in a pot

It will, but they're not great container subjects - there's a smaller, dwarf version that might be worth looking for - this seems to cope better with the containment. Gets about 3/4 feet high, from memory, maybe up to 5.

Davew, i've grown Buddleia in clay soil and they were okay, maybe have a go ? :)

If you have little room, why are you thinking of planting a Buddleja anyway? They can get huge and quite unsightly very quickly. Is there a smaller plant that would suit your purposes instead?

I have 2 Buddleias both in tall deep pots about 2 foot from the house wall. They've both done really well. The pots are about 60cms high and about 45cms across. The buddleias need quite a lot of water and feed, I use tomato fertiliser but I alternate with general purpose stuff. This is the first year I've had them, very successful so far and the butterflies have arrived in legions!

Larger Buddleja cultivars will only last a few years in a pot, even a large one. The solution is to grow a new one from cuttings about every third year. Another solution I am experimenting with is to remove the plant each year, prune the roots back a few inches and then partially replace the compost. Looks to be working, at least if done in August after flowering. However, late spring is probably the best time.
Thompson and Morgan's Buzz series are bred for containers but they haven't been out long enough to assess whether they are suitable long-term.
Remember that the pot will need to be sheltered in winter as it is much more exposed to cold and water-logging than the soil would be.