Answer from Steve Dawson - Britain is the slug capital of the world. Nowhere else has such ideal conditions for a broad range of slug species, our cool, damp summers and warmish, wet winters allow slugs to be active, breeding and feeding for so much of the year. Added to this, our recently succession of milder winters, and cooler, wetter summers has helped increase the slug population. Normally icy conditions kill off a percentage of slugs and prevent the remainder from breeding and eating our plants for a while. Similarly, hot, dry weather kills slugs, which have to burrow deep into the ground to find the moist conditions that they need to survive.
On the positive side, slugs can help cycle organic matter, which in turn helps to build healthy, rich soil. Also, they are a source of food for wildlife, which we should welcome into our gardens. I would not encourage slugs in your compost heap because they could breed and can then be spread around your garden with the compost to then munch on your plants.