Develop Post Code search to locate nearest office

We are looking for a web based “Nearest office” feature..
We have 40 offices around the UK..
We are looking for a way of displaying the nearest office to a post code.
The post code will already be in our database and we just need a web page that can generate the result by including the searched for post code on the URL.
The Post code will just be the Out Code, ir DE22, or DE 23 not the full post code.
For example, and really just an example of using URLs to search for web pages and nothing to do with post codes..
If I wanted to search for the people per hour Facebook page and had the ID I could just add it to http://www.facebook.com/profile.php?id= to bring up the page…
OR
W are happy to look at a more enclosed system, possibly using Google Maps or Bing maps.. Possibly plotting the 40 offices on a Map so also visualising the nearest office, or giving an option to the customer of more than one office.
At the end of the day we want to quickly identify which office is closest to a postcode and will take advice on the easiest solution.

Here are a few examples of what we need..

http://www.rspb.org.uk/reserves/area/postcode.asp
http://sendsocial.com/delivery_partners/bybox/

And here could give you some ideas and resources to help you do it..
http://www.freemaptools.com/download-uk-postcode-lat-lng.htm