Spiraea - should be pruned immediately after flowering.

Choisya - should be pruned in spring.

The RHS website will tell you all you need to know about the pruning of any plant.

If you prune them now, it will stimulate them to produce new foliage which could be damaged by frosts in the coming season. They probably wouldn't flower next year. However, sometimes you have to prune at the wrong time for a variety of reasons. It's not likely to kill the shrubs, just set them back a bit.

In my experience: although the ideal pruning times are as Rosierose says above, leave the Spiraea until late October and do it then (but if its a spring flowering one such as S. argutea, you'll lose the flowers). You won't stimulate growth at that time of year because the shrub is going into dormancy.

The Choisya I wouldn't risk pruning back at all before next spring, unless you need to for something crucial like building work and there's no choice. If you must, you must, but again, best left till late autumn if you really have to do it before Spring.

Thank you for your advise. I can wait with the choisya but will trim the spirea back in October.

Regarding the choisya I would wait until the spring, don't be afraid to be ruthless, if your choisya is to be renovation pruned it will respond very well indeed, let's say if you have one that is completely overgrown then you can cut down to virtual ground level and with a few buckets of water it will come back very quickly.