The Royal Mail's online Post Code Finder is a useful tool to know about if you already have most of the rest of a British address.
Enter some basic information - house name or number, street name and town and, as long as you've entered that information accurately, the tool produces a full post code and a map of the location. There are also some tips and pointers to help you search.
City Addresses are trickierIf you are heading for a larger city, where there will be a lot of streets with the same name, it is still wise to keep track of the postal code before you leave home - or at least find out the name of the neighborhood you're heading for. Otherwise the tool may throw up six, ten, even twenty different addresses. Find out why knowing a British Post Code is vital for getting around the UK
Correct Format for a British Postal Address
Don't forget, if you are about to send some presents to friends and relatives in the UK, knowing the British Post Code and understanding how to format it and where it goes on an address is essential. Here's a sample (completely made up) of how to do it. The Post Code is indicated in Bold Italics. If you have the city, the county, and Post Code in the correct places, you don't need to indicate England, Scotland, Wales or Northern Ireland. Just using "United Kingdom" is sufficient. For big cities - like London, Liverpool, Glasgow, Edinburgh, Cardiff, Swansea, etc - you don't even need to include the county, just put the Post Code after the city name, with no commas. So here goes:
12 Oak Street
Little Cheaphampton-nr-Big Bottom
Kent XY5 12UZ