The very best jerk seasoning in the world is a wet marinade called Walkerswood, which is fairly easily available in many London stores including Tescos. If you can't get access to this, make your own. It's easy, and you will find your own style with trial and error. Here is a starter recipe to get you going.
1 onion, finely chopped
2 teaspoons fresh thyme leaves, finely chopped
2 teaspoons salt
1 teaspoon all spice
¼ teaspoon ground nutmeg
½ teaspoon ground cinnamon
1 teaspoon sugar
4 to 6 hot chilli peppers, finely chopped
2 teaspoon ground black pepper
Mix all ingredients together in a bowl. Add a little water to the mix to form a paste. Rub onto your chicken, pork or fish and marinate for up to 24 hours before cooking.