You can usually invent a post code that is valid. All major post offices have a suffix of 1AA. For the prefix, you can usually use the letters from your prefi

You can usually invent a post code that is valid. All major post offices have a suffix of 1AA. For the prefix, you can usually use the letters from your prefix but change the number to a 1 and if applicable any letters that follow the number to an A. So if your postcode is WC2E 9AB then just give WC1A 1AA. Voila, valid post code for the main post office in any area. Edit: This is legacy, a lot of the 1AA post-codes were recycled in the 1960s when the post office underwent major re-structuring. It still works though.