Ok, this is my personal way of doing it.
Since I'm not that fond of the XML parsing in Groovy (read: because I'm lazy), I would make a Properties Step containing properties for these:
Request PostCode
Response PostCode
I would then move them into the properties step using a Property Transfer like above (only now into the Properties Step) and the accessing the Properties Step from Groovy.
def step = testRunner.testCase.getTestStepByName( "Properties" )
step.getPropertyValue( "Request PostCode " )
step.getPropertyValue( "Response PostCode " )
In Pro it is simpler to use Grovy and I would use the GroovyUtils like this:
def groovyUtils = new com.eviware.soapui.support.GroovyUtils( context )
def respHolder = groovyUtils.getXmlHolder( "GetUKLocationByPostCode#Response" )
def reqHolder = groovyUtils.getXmlHolder( "GetUKLocationByPostCode#Request" )
reqpHolder.getNodeValue( "//web:GetUKLocationByPostCode/web:PostCode"))
resppHolder.getNodeValue( "//web:GetUKLocationByPostCodeResult/Table/PostCode"))