Need more information re its growing conditions - is it in a pot or the ground? In sun or shade? How long have you had the plant?

Feed me, Seymour! Feed me.

It will be exhausted if it has flowered heavily. Nitrogen, iron, magnesium and potassium will all be depleted hence the white leaves. Buddlejas are greedy feeders although the myth is that they like meagre conditions.

For quick results foliar feed with dilute liquid feed. Otherwise any soluble feed with trace elements.

If it is a different problem the leaves would more likely droop and go brown rather than white, which sounds like abscission.

Although I agree we need a little more information on growing conditions to be sure.

Even with abscission, Buddleja.., the leaves shouldn't turn white. I'll be interested in the cultivation information... and also, quite what part of the UK Katb23 is in.

I was thinking classic chlorosis, but I agree white rather than yellow is a bit alarming. In any case feeding minerals might help.

I know that they can withdraw nutrients from the older leaves when they are nutrient deficient, which they then can shed. Really common this time of year but it varies by cultivar.

A dying plant does not usually manage to withdraw the essential elements from the leaves.

Another very rare phenomenon is the loss of all the green chloroplasts (I have a photo of this somewhere) but I have only seen this in variegated forms.