I usually buy my dried chilies online from cool chile (who are great) however I've also recently found that the Sheeplands garden centre farm shop on the A4 between Reading and Maidenhead sell various dried chilies including ancho, chipotle, passila and bhut jolokia. They also sell mexican oregano as well as a good selection of artisan hot sauces and condiments.
Worth a visit for anyone in the area.