http://www.google.com/apis/maps/documentation/#Geocoding_Examples
but GClientGeocoder doesn't work for UK postcodes

You may be able to use the Google AJAX Search API in online apps. This does work with UK postcodes apparently but I'm not sure about the format returned without looking into this further ... http://googlemapsapi.blogspot.com/2006/08/how-to-add-google-search-to-your-maps.html

See here for more...
http://mapki.com/index.php?title=Read_This_First#Geocoding_UK_or_China