It was becuase orderId is using default namespace.
Here you go:
Declare the namespace for "
http://harlequins.bt.com/schema/OUKMessages" with a prefix
Then access the node using that prefix.
Following Sample groovy script placed after soap request test step whose name is Request.
def groovyUtils = new com.eviware.soapui.support.GroovyUtils( context )
def holder = groovyUtils.getXmlHolder( "Request#Response" )
holder.namespaces["ns"] = "http://harlequins.bt.com/schema/OUKMessages"
def orderIdValue = holder.getNodeValue("//ns:orderId")
log.info "Order ID from response : ${orderIdValue}"