They are directions to somewhere near 18 Whitehall, London. 18 Whitehall is a real place and the post code is not complete but SW1A 2 is the correct partial code for that area of London, it just needs 2 letters on the end to make it complete.

You can look at that street on Google Street view t see if you can find the place you are looking for http://goo.gl/DyNN2