You may have slugs but it looks like more than that to me. It may be some viral illness or just stress from the drought. Choisyas do need regular refreshing to keep them looking good. I'd be inclined to cut it back really hard and take out all the old wood, leaving just the fresh new growth. It will soon bush up again. If you do it now it'll still have time to to put on some more growth before the Winter.

It looks more like heat and drought damage, to me.

I was going to say...I seem to have most of the slugs in the county of Angus living in my garden...but they have never done this to my choisya. I’m wondering about fungus and also drought...possibly some damage low down on the branches?or perhaps a flying pest. There are so many insects that will strip tree and shrub leaves.

I was thinking drought because of the way some of the remaining leaves look badly sunburned. If it is drought, you would find the missing leaves under the bush. If it is bugs or slugs, some of the remaining leaves will have irregular rounded parts missing, the missing leaves will be completely missing, and there will be copious amounts of tiny black, dark brown, or dark green granules, called frass, about.