Enter your postcode in to the post code box at www.streetmap.co.uk and click Go, under the map click link to convert coordinates to get your Longitude/Latitude
Go to the Amsat UK website and access their Long/Lat to IARU Locator Converter which will calculate your IARU locator and display it.
To work out your Worked all Britain Square look at the LR reference (Same location as longitude/Latitude)
You will see a Alpha numeric sequence similar to this one SU507670
The Worked all Britain reference is formed by the Two Letters and the first and forth numbers.
In this case the WAB Square will be SU56