As far as I am aware churches do have postcodes. The church where I was minister certainly had a postcode even before it had a letterbox, and I have sent letters to a number of churches in England before advertising various conferences. They all had postcodes. If they don't have a letterbox, the postoffice usually had some arrangement with them. You should be able to find postcodes online. I can't remember the website for UK postcodes right now and I'm busy working on something right now. Try a goodle search on www.google.com