LIVERPOOL, England, Dec 12 (Reuters) - Britain is considering all options on how to respond if Russia invades Ukraine, foreign minister Liz Truss said on Sunday, highlighting that it has used economic sanctions in the past to send diplomatic messages to Moscow. "When the UK has wanted to send clear messages and achieve clear goals we have been prepared to use economic sanctions," Truss told reporters at a G7 foreign ministers" meeting. "We are considering all options."