Re the mildew on the osteospermum, depending on what variety it is, it'll die over winter anyway, there's only one that makes it through outside in winter here, and if yours is hardy, do not cut at all now, leave it as it is for the winter. For future interest, mildews like this can be treated with a spray fungicide, or 1 part milk to 9 parts water mixed in a spray bottle and spray till run off with that.
I did see the pic of your Camellia and I did think virus - but most plants with a virus not only display yellowing of some description, but often distorted or twisted leaves, and there appeared to be no sign of that on your plant. I also agreed that it doesn't look like chlorosis from alkalinity, so perhaps you bought a variety that happened to have variegation on the leaves naturally? Interestingly, variegated leaves on plants came about through viral infection in the first place, and the breeders bred from those, since the plants were healthy other than the coloration.
As for the Skimmia, if you are saying there are some leaves which have gone yellow, this might just be natural leaf loss - despite being evergreen, they do drop leaves throughout the year rather than all at once. If the plant is suffering from alkalinity of the soil, it should look pale, a bit sickly with yellowy green leaves all over. So if you just have a few bright yellow leaves, but the rest is green and healthy, don't worry about it.

Was there a label on your Osteo? The hardy one is O. jucundum. (NO name after that).

The yellowing also looks like it may be chemical damage. Are the shrubs near an area where they may have been accidentally sprayed with light oils (gasoline, kerosene, etc.), or with strong cleaning fluids (vinegar, ammonia, power washing detergent, etc.)?

To Sprithenry

It is a Stardust. It is a hardy variety as it survived last winter and has produced a huge amount of flowers this summer and it still flowering.

To Tugbrethill

Thanks for ideas but there is no way any of the fluids etc that you suggest have been used.