Spring Integration: How to get value from util:map into SI's Router

My configuration is like below: I am using SPEL inside router to get values from map.

        <util:map id="routeConfig">
        <entry key="Default" value="not configured" />
        <entry key="GB22XXX" value="LON" />       
        <entry key="AEADXXX" value="ME" />
        <entry key="HBXXXX" value="ME" />
        <entry key="EBHBXBAO" value="ME" />
</util:map>
<router input-channel="InputRoutingChannel" default-output-channel="testOutputChannel"  expression="@routeConfig.get('payload.getMsgHeader().getSourceSystem().substring(4, 12)')">
        <mapping value="LON" channel="MarshallerOutputChannel_lonme" />
        <mapping value="ME" channel="MarshallerOutputChannel_me" />
</router>