Deutzia is one of the shrubs that should be pruned immediately after the flowers fade. You can take a chance and cut them down now as you're near the sea, but you likely won't get any flowers next year or the year after. This sort of renovation on old shrubs is usually carried out in winter, when the plants are dormant though. Otherwise, wait till next year, let 'em flower, then cut back hard after flowering, but if you're going to do this, go out and take down a third of the shoots to ground level in winter, when the plant is dormant, if they are very congested at the base.